Here's a question I got to wondering about recently. In the Middle Ages, with murder being a capital crime punishable only by feudal overlords, how do the mundane nobility and clergy feel about Wizard's Marches? After all, renounced magi won't necessarily have committed crimes in the eyes of the church or the crown, and yet other magi claim to have the authority to hunt them down and slay them out of hand. I daresay a lot of lords temporal and spiritual would be happy to let magi police themselves, but what would happen if a covenant or tribunal came up against a noble ready to treat a Wizard's March just like a murder committed in their area of jurisdiction?
A noble trying to hunt down a hoplite is a nice twist in the story. Especially since messing with the noble or his soldiers can be interpreted as meddling with mundanes. This might give those unhappy about their friend being marched the opportunety to payback. Maybe it was their idea from the very beginning to tip off the noble to start the hunt in the first place....
One must consider, of course, just how willing the nobles (and more importantly their men-at-arms) are to track down and detain a person who can literally kill with a word and/or throw fire from their hands.
In my saga periodic wizards wars, and the lingering memory of the Schism War, are enough to remind most nobles and clergy just why it's a good idea to let the wizards police themselves.
I also decided for my saga, because of possible situations like this, that the Code actually holds the weight of law alongside mundane laws. The Order is recognized as having legal authority over practitioners of magic and the Code is the law that they enforce. Just as clergy are subject to Canon law, so are wizards subject to the Code.
Of course, other saga's take a completely different view. My Order is much more open and "in the world" than is probably common.
Of course:
a) not all magi are actually super-effective at this sort of thing (although probably one who has successfully prosecuted a wizard war is).
b) the noble and especially his men-at-arms might be rather vague about exactly what a magus is capable of...or even that the murderer they are pursuing is a magus at all.
... and thereby hangs a tale. One I might have to steal for my saga! 
Been there, done that. Amazing what the thought of killing the dudes had on my players when remineded about the fact that they are totally innocent and that they are being a murderous bunch. Funny, since they had just massacred a whole highlands clan that was aiding davnalleus without even blinking 2 weeks before, but hey 
It started a nice "we should gert some non lethal spells and maybe go on a pilgrimage" movement IMS 
Cheers,
Xavi
More militant Magi would say that the Code prohibits them from interfering with Mundanes yet does not then allow them (Mundanes) to interfere with Magi either. In other words they should still have the right to self defense if you will.
Conviction at Tribunal is more largely a political matter than a legal one.
Some of my characters in the past would have obliterated or 'brought to heel' the pesky noble out of hand without a second thought. Some wouldn't have. Either way generates a story.
A related question is what would happen if a renounced magus were to seek the protection of a noble or priest, in exchange for making themselves useful, e.g. someone that magi would normally be careful not to antagonize?
A story!
There's no one answer to this. You tell us what happens.
Yeah, thats a story opportunity if i ever saw one!
Well, I already have a renounced diabolist Quaesitor who has gone to a monastery, confessed his sins, promised never to cast spells ever again as penance and become a monk. He has the full protection of an abbot who has decided that Canon Law takes precedence over the Code of Hermes, and that no one is going to be killing anyone if he has anything to do with it. This has kind of left the Quaesitores in the Tribunal wondering what their next move is.
This is where veil of invisibility and spells like curse of the desert, clenching grasp of the crushed heart, Grip of the choking hand/winter's icy touch followed by pillow on the face, or black whisper with a subtle/silent caster comes in.
Let him die a natural seeming death. Alternatively, cause a beast or something to attack and kills him and you insure there is the scent of sulfer and brimstone about his quarters. his sins brought the devils down upon him.
Indeed. But the executioners would still need to bluff their way into the monastery, or find arcane connections, and then overcome the Divine Aura to cast the spells. Furthermore the Abbot is by now somewhat wise to the ways of magi so no matter how natural seeming the ex-Quaesitor's death might be, he is still likely to be extremely suspicious. It could be very easy to discover the truth in a Divine Aura with a Just Temper.
Bah. Not as a matter of course.
Now if the abbot was a practitioner of divine methods & powers things might be different. But if not... Pfffshh.
Summon a nasty faerie, let him tear the place apart, and then put him a bottle. Let the fae take the wrap and lay low for a while with the concubines.
And as long as it happens within a Dominion aura, who knows what divine interference there might be("something" might think its really great to have a repenting diabolist around for some reason), just think of think the problems it could cause if the spells to kill fizzles out and lets say cause a lightning bolt to strike the "offending magi", now that would be one serious case of getting the church "not happy" with the order!
Put spell in a charged magic item so that it does not have a die roll and aura doesn't make it go sour.
night with veil of invisibility should work. Sure the abbot would be suspicious but he would be suspicious no matter how guy dies. Demons would help order out for no charge most likely to serve as lesson about betraying hell and repenting.
Sure, the demons are probably willing and able to help.
But, receiving the aid of demons to march an infernalist somehow seems problematic to me.
No one said it was a good thing. Dealing with demons is bad but you are marching someone that broke the code for which punishment is death.
Yes, you are breaking code to kill him but you have to get caught it. If demons kill him, there are not likely to be repercussions upon the order for his death which is what this topic is about.
Of course there is always the charged magic item as I said, mastered spells and all sorts of other ways to kill him (including having an mundane agent of the covenant hire a mundane assassin) and leave false trail to some other mundane.
The issue is that you have to be subtle in solving this because of interference with mundanes when this sort of situation comes up.
Precisely, the argument that any demon would make, I guess.
Why would the demons not arrange for you to be caught?
Apart from the fact that this abbot becomes convinced that The Order is in league with with demons?
Which it, effectively IS... 