How is evil defined within Ars? If you wanted to be obvious about it you could choose to play a diabolist or a mage who is attracted to the infernal, but what if you're just an asshole? Is that evil?
What I'm trying to say is: How am i supposed to learn to turn people into birds if i dont practice on some people? Is that evil or is it just evil after the cat eats them?
To begin with I'm not completely sure I understand the question.
I don't really see the Ars publishments saying much about capital 'E' Evil. Sure you might emphazise the infernal realm etc. Or reading the RoP:D get the impression that according to the faiths just the act of using magic was quite condemning. BUT. Basically defining what is 'evil' can be hard in real life as well as in a RPG - and it is very dependent on who you are. And if you believe in the concept of evil in the first place...
Basically I think that what I'm trying to say is that the answer to the question is at one time both very relative and very firm. Relative because we each have various concept of evil, firm because if you believe in evil you tend to be quite firm in what you believe to be so. So basically if you seek an answer to the question of what is evil in Ars Magica - no one can really answer that but yourself and your troupe.
For a further discussion of how Evil was viewed in the middle ages by Christendom, you might read Thomas Aquinas, or St. Augustine.
For a discussion of the Divine Realm in Mythic Europe, read "Realms of Power, the Divine" (RoP:D for Short)
There are necromancers that practice their art. By necesity they must disobey the laws of Christendom, and practice on the dead husks of the departed. Is this magus Evil? Christendom would say yes. Does he think he is? Up to him. His player might play him as tormented by the ghosts of his victims. Does this give him feelings of remorse? Again, up to his player....
Hope this helps.
eric
In game , you are definitely Evil if you willingly serve the Infernal.
(God tells me so)
Anyone with the Gift is an arsehole in a sense , by default of having it.
You will be considered capable of all manner of depravity or other heinous acts by those you interact with.
(Gentle Gift the obvious exception)
For the most part , the Moral Code in Game , is based on the principles of the Catholic Church.
If you are at a Covenant , you can always "volunteer" Grogs to be test subjects for your Avionics experiments.
Deliberately letting a Grog get eaten by a cat , while in bird form , is definitely a mortal sin.
Unless you have a positive Pious trait or have a virtue/flaw relating to The Church ,
you may not care about such things.
You could take a negative Reputation , Infamous (page 55).
People would consider you evil because of this.
Magi are lacking in the Social Graces when dealing with non-Hermetics.
Being an arsehole is probably par for the course.
In and of itself , that is not inherently evil.
Thank you all, those are good answers!
I don't think 'evil' is defined within Ars Magica. So if you turn people into birds, you'll probably won't consider yourself 'evil'. The people being turned to birds might.
The real issue is what your Storyguide thinks of things. In particular, what he thinks are the ramifications for your actions. It may be that by commiting these acts you are, in fact, comitting acts that condemn your sould to hell. The devil may take a special interest in encouraging your actions (perhaps arranging for you to find a certain spell, or a mysterious magus that will teach you some Muto...), in twisting your actions in ways that will lead to your damnation (perhaps arranging for a cat to eat the bird, for example), in spreading your practices to other magi, and in arranging for your untimely death (so that your sould won't escape his clatches into Twilight).
Your storyguide may base his decisions on such matters on anything. It's really up to him, so you should ask him. If he goes by the official rules, found in The Infernal, then IIRC an act a sin if you know it's wrong and yet you do it; this means that the decision whether something is 'evil' or not is really up to you as a player, deciding whether your character knows that what she's doing is bad. Aiding the Infernal is always a sin, regardless of any your decision as a player. Again, however, it's best to ask your storyguide. He's the only one that can rule what's a sin in his saga.
I think this might be a case of "if you have to ask, chances are..." or the other personal favorite, "Would it be acceptable to tell your mother about it..." Carnal matters aside, chances are if Mom won't approve, it's probably not very nice. 
Especially if you need to rationalize or say "but it's only ... "
It may just be a matter of how terrible a person the magus is...
-Ben.
We have been actively exploring the issue of evil IMS. Two of our characters are reasonably devout Catholics and one is desperately trying (through good works and leading an upright life) to correct the sins of his youth. The other one tends to rationalise his behaviour and probably wouldn't be considered evil as much as extremely selfish. His recent murder of someone who was blatantly a saint probably hasn't helped his chances of escaping damnation though.
The third character, my own is clearly evil, he has no real regard for human life and is a praticing necromancer. Mostly however he just wants to be left alone. His evil comes from his lack of empathy for others so he might be best described as psychotic, although psychopaths generally have little ability to foresee the consequences of their actions (or lack of care for those actions) whereas this guys a plotter.
Personally I do not believe in the concept of evil. Well, more or less - having sort of studied the genocides in Rwanda and Balkan have at times made me doubt that stance. But basically I believe in callousness, in the failing of humanity, and in personal failings of ethics feeding a sense of having done 'evil', but I do not believe in 'Evil'. Simply because it seems to me to rest upon a notion of a consciousness or 'actor' - either as an instigator or as the firm concept of 'not-evil' that evil has to mirror.
[EDIT - and the above is why it is just as hard for me to address the subject in a roleplaying game.]
Even so, and even if being elusive, bound by relativity and a possible mine-field, I think that working with evil in a roleplay can offer some interesting stories and rewarding scenes. Not so much as 'go-into-the-cave-to-slay-the-evil-antagonist' evil, but as moral dilemmas and regrets of characters themselves experiencing having done or about to commit what they at their core feel to be henious. This was one of the things I found to be intriguing about Vampire when it worked the best (before the whole game became to much about revelling in the godlike macabre or playing at shallow pathos).
However, it is a difficult path, even if rewarding, but to pull it off requires a deep understanding and an even deeper trust.
I'll admit that my own views on Evil are flexible. Morality is expensive and, as real life shows at the moment, is inclined to be thrown out when it gets too difficult. No one expects those in a serious fight for their life to show mercy, theft is a perfectly acceptable alternative to starving because you cannot afford food, and most civilians in a dark ages land conquered would rather become slaves than be executed as enemies of the victors.
In the context of mythic europe and morality the best solution is that evil is what is proscribed by their society. For Christians evil is what is banned by the Bible, for Muslims it is what is the Koran forbids. Pagans are harder to deal with in the Ars magica game because we don't have a real text to refer to for their code of conduct. Certainly if their gods demand sacrifice it would not be evil to carry it out. However that runs into difficulty when you consider in game terms that the entity worshipped might be a false god.
If we are talking about game terminology and making the decision on whether someone has sinned and will end up in hell, then make up a subject set of rules that you think summarizes the attitudes of hell and heaven as what is right and wrong. Certainly deliberate worship of the infernal is wrong, as would be necromancy as it would interfere with the rightful flow of souls to their fate.
I am inclined to the view that mind control magic are no worse than forcing someone through other means to obey you against their will. That mind reading is no worse than a brutal interrogation. That assualting someone with magic or even killing them is no worse than using mundane methods. In all these cases the context is more important than the action, doing it out of necessity and to protect others is okay, doing it for fun and to show off your power is not.
On the question of magically experimenting on mundanes, I would say that it's slightly wrong, try and only experiment on condemned men - criminals etc. or volunteers. If you think volunteers is an impossibility then just think how many people volunteer to be guinea pigs in medical trials in real life. In a medieval context I am sure you will find many beggars in big cities willing to move to a covenant and be test subjects in return for regular food, and a warm place to sleep.
Certainly don't experiment on vassals of the covenant who have sworn obedience and can expect protection.
This is all wonderful insight. I have barely begun to play this game and i already love it.
Related to the subject of evil, I found it interesting to (as a storyguide) roleplay demons and devise their wicked little plans. By definition, demons in ArM are purely and unredeemably evil. It made me think about what evil really is, and how many different kinds of evil there are.
Indirectly, this sparked my interest in ethics in general and led me to read rather more theology and philosophy than I otherwise might have.
[Edit] And, of course, once I really understood evil, I embraced it wholeheartedly. 
Yet another Infernal ploy exposed: Ars Magica as the Instrument of the Devil.
Just don't tell Jack Chick.
I hear you. In my humble opinion, the single greatest thing about Ars Magica is that it pushes you to read more about history, and in turn more of the great thinkers. The things I have learned are amazing, and my life is so much more rich because of it.
One of my fellow players, when he was storyguide, began a deep and emotional search into the religion of his youth: Catholocism. After researching many aspects of the Church for his game, he found that he had a love and respect for it that he had formerly dismissed. He then began exploring It from a spiritual point of view, and moved on to Thomas Aquinas. (Anyone who has attempted to read his works knows what heady stuff this is. It's beyond me....) In the end, he's returned to the Church, with (I think) a much more meaningful understanding of what he is practicing than his fellow parishoners.
In no small part, Ars Magica was part of his personal growth.
I love this game.
Eric
I like the concept that good or evil depends on who you ask except for this one thing.
Lucretia Borgia probably didn't define herself as evil. She was well behaved and raised properly. She didn't make arm farts. Probably none of the Borgias did. She was a murderer and she fucked her brother, but her dad said it was okay, apparently, and he was Pope.
It's no good to define evil by whether or not your mom would approve or whether the church agrees with you.
But everyone else defines it that way and thats what will get my character burned at the stake if she's not careful. She doesnt think she's evil, but if she eats entrails of some freshly killed beast and shows up in human form with blood on her face, people might get the wrong impression.
( BellaDonna is a Bjornaer raven)
So I think I understand now.
HAHA
I told you I am not ruining the game by revealing too much about my character in here! Theres plenty to reveal.
Really, I am just trying to understand her better.
oh i see now you meant lucretias story. haha im an idiot. 
And why shouldn't you!!!!?
And why shouldn't you too!!!?
BRING.HIM.ON !
And if you are in doubt, I'm not talking about sticking a bit of Ars up his "Dark Dungeon" -which would be sacrilegous in so many unspeakable ways- but of all the love this magnificient game seems to be fostering as of recent. And we all now that love is the kryptonite of hatemongers!!! 
Jack Chick is evil.
Is that with a little "e" or a capital "E"?!?!?
And if by evil, you mean HILARIOUS, then yeah. He is.