Also keep in mind: The Hohentwiel sits right on the tribunal border between the Rhine and the Greater Alps tribunals. If you stick to Sanctuary of Ice, there's a covenant of Flambeau (Juno's Spire) with a battle-hardened archmagus (Pietro ex Flambeau) right across the Lake Constance. There is also an unscrupulous Tytalus quaesitor who is (in)famous for patrolling the borderlands, making sure there are no infractions against the resources of the Greater Alps tribunal.
My first thought is that a mass murder on this scale would probably create an infernal aura, so now you have to worry about the clauses involving interfering with the mundanes as well as messing about with demons and also have to worry about what sort of infernal entanglements you have just created...
and a demon showing up disguised as a quaesitor will really make your day difficult.
Yes. Yes it is.
What you're discussing is the sort of thing that Infernal auras are born from.
-Ben.
Where's the "+1" button?
This is a great point. I like it! 
-Ben.
Which isn't a crime under the Code in and of itself. It could however sustain a case for infernalism brought by your political enemies. And that, too, is a story.
Isn't the precedent Curse of the Unportented Plague (PeCo 55, ArM5, p. 133)?
It is noted as "strongly discouraged" but is not forbidden.
Not forbidden, but even transcribing it is discouraged, and again while the spell itself may not be illegal individual uses may be prosecuted, depending on the results.
Is this going to be a new covenant? Getting permission for a new covenant is hard in the Rhine Tribunal, and enough of the other covenants might find your actions distasteful that they wouldn't allow the covenant to be created.
Are you sure that the competition for the site from other magi wouldn't be too much once you cleared the mundanes out? If this neat, nifty site opens up, a lot of magi would try and gain its resources. The idea that you are the one who killed all the mundanes and thus the site should be yours is probably more of a strike against you in the tribunal.
Interesting. 
This rather looks like a Jerbiton intrigue to take it over - perhaps with the help of some magi versed in Tribunal politics.
Cheers
That's what the Order previously did to gain access to the site in my saga. But then I threw in the classic "the old covenant mysteriously vanished and you were sent in to investigate" trope. 
I've been thinking about writing up the details of my campaign for this forum for a while now... maybe this gives me the motivation to actually do so.
Some might want to have a look at chapter 8 of Hooks.
Looks like it could be relevant.
</SPOILER ALERT>
"The River"?
Oh, and a final nitpick: The Hohentwiel is not a dormant volcano, it's a mountain of volcanic origin. I figure that'd make it slightly harder to erupt. 
Does anyone have Hooks yet?
I don't think so, (except maybe the authors) but the intro posted here suggests that #8, The River, might bear some similarity to the situation being described in this thread.
-Ben.
That is the sort of thinking that condemns the character's soul to hell. You do realize you are playing an evil madman here, right? And you understand that this is a game with a definite black-white morality. God is real and so is the devil.
It is really up to you and youyr SG, tell the sort of story you want. But if it was my game, I would find a way to drop a sack of hammers on you.
But take a step back. Do you really want to play such a horrible evil character, unrepentant and unpunished? Is it your desire to play a game where everyone can act out their evil fantasies without consequence?
That is kind of dark and uncomfortable.
And take no comfort in Philipius Niger. I doubt that he would be that heartless and evil. Dominion and power? Sure. Cold blooded murder? I doubt it.
And Philipius Niger is sorta weak compared to most Archmagi, and any well trained ten-year Flambeau could wipe the floor with him. His enchantments and wards have zero penetration, his scores are low, and I really doubt he would back you on this one.
The kind of 'it isn't a crime if I can get away with it' thinking is all too common among magi (and people in the real world, alas) but won't prevent the consequences from finding the PC. His new covenant will be infernally tinged right from the start and all sorts of Very Helpful People will be along to offer their assistance and advice and gradually lead the character into the service of Hell, not that he has far to fall. Once he is consciously accepting demonic assistance then he's guilty of infernalism and that's the one bit of the Code that's not going to be ignored on political grounds assuming he lives past attentions of Quaesitors, Hoplites and visiting warrior angels. I wonder if the whole group will find fun in the Saga of his damnation?
And why murder at all?
Surely, the way to do this, given that reactivating a long dead volcano is going to be a massive expense in vis and tricky with it is to do it in stages.
First, go up there and start the volcano on the route to reawakening. A few earth tremors, much venting of steam and smoke and the first tentative signs of the rock growing hot. Then a small scale eruption of lava.
By this point both nobles and commoners are going to be starting to worry. Arrange for a 'well-known expert' in the field to come along and offer his counsel to the afflicted fief holder. "Alas my lord, I fear it is worse than you think. I have seen these signs twice before and they invariably mean that the fires beneath the earth are going to return in full strength. If I were you, I'd sell the place off for what you can get..."
Now, this is fraud and theft and liable to put the character's soul in danger but not so likely to corrupt the very essence of the place. And much less likely to lead to the attentions of Hermetic justice, though you might still get demonic interest in the soul of your character.
I think you've all missed the part where I said that I would be planning this a long time down the road.. of course I wouldn't dare commit such an act of hubris at the start of my career, when I am at my most vulnerable. I plan to take things slowly; to make allies and amass power. To increase my parma to godlike levels and fine-tune my laboratory into a terrifyingly efficient machine. I have the next 160 or so years, after all. When I make my move for that mountain, no one will be able to stop me from doing so. I am planning my rise as the Dragon King. At 24, I am already nearly as talented in CrIg as Flambeau the Founder was.. give me a century or so and I will really start to shine. The mountain is honestly one of the smaller parts of my plan.. and won't take place until after I become archmagus, and after I found my own path. When my covenant grows in size enough there will be none who would dare stand against me. I will become Artorias Draconis Rex, and you shall all bow before me 
edit:
Also, I've decided just to pay the entire town to go away. Through some.. shady dealings we've amassed several thousand pounds of silver. I couldn't care less about those little mundane trinkets, but one of my covenmates insists they are necessary when dealing with those lesser beings. I would prefer not to deal with the scum at all, but then the world needs ditch diggers, am I right?
(Overheard at Tribunal)
Primo Magus: "You know what I've always liked about Artorias?"
Secondo Magus: "Other than the cookies he always brings?"
1M: "Yes, those roasted walnut oatmeal bites are delicious, aren't they?"
2M: "They are. He has been a pillar of tribunal. Regularly contributes work for the Folio. Excellent insights into terram."
1M: "And just made his Archmagus last season. I sent him a congratulatory gift via redcap."
2M: "That's so thoughtful of you!"
1M: "Least I could do, after he helped out with that bit of trouble with the troll."
2M: "Quite. I'll have to send him a note."
1M: "Yes, so besides the cookies, I've always taken a liking to his consideration and level-headed thinking."
2M: "Hard to find that in the Order sometimes. There's always someone planning to make a massive storm, or manipulate the local lord, unleash terrible disease on the populace."
1M: "You'd think they'd missed the fact that the Divine is always watching. Omnipotent creator deity and all that."
2M: "A bit too much Warping and Twilight sometimes. Oh, and" (gestures pinched fingers to pursed lips) "too many addled seasons! Even a wizard forgets."
1M: "Yes, well, thank Providence we need not worry about that from Artorias."
...
My cookies will be feared as far as flows the Rhine!
I'm not sure what the Divine has to do with anything, as I've decided not to murder the pitiful creatures living at the base of my future throne.. and I don't plan to murder anyone who doesn't challenge me. As far as I can tell everything I want to do is perfectly within the limits of the Code of Hermes, wouldn't be considered 'evil', and doesn't involve the Dominion at all- sure, some people will have problems with me consolidating power.. but what are they going to do about it, exactly? I simply plan to become the most powerful among the magi. Surely I'm not the first to aspire to such a goal?
And by "bow before me" I don't mean "worship me as a false idol", I mean they will respect and fear my power. You must think me foolish indeed to tempt the wrath of the Divine.