Believeable Character Development?

This is one of my all time biggest weaknesses as a writer (which is Ironic, seeing as I hope to be GOOD as a writer in the near future), but the characterization would go for my Antagonist collecting the 20 Weapons as asked in the previous thread (thank you A Ll for the ideas by the way. it's greatly appreciated).

However, I want my Antagonist to go from a "Well Intentioned Extremist" to a "Complete Monster". But god help me if I can think of a way from someone who goes from "Hey, this is for the good of everyone" to "Fuck everyone, the world BURNS."

Do you think it would be too much of a leap from one end of the Antagonistic Spectrum to the other?

Villainous Breakdown.

Basically, a Trauma Conga Line for your villain. He starts out meaning well, then associates himself with some shady people (these are the only ones who will support him. Too bad they're Complete Monsters and he has to kill them . . . but not before they ruin his initial plan all to hell. The one that would have saved everybody.

Okay, Plan 2. If his plan succeeds, some people are now fucked, but that's okay, because he can save the people he cares about. Except, when the opposing government/anti-heroes find out, they use them as bargaining chips to try and coerce him to abandoning his plans and setting things back to normal. Shit goes horribly wrong, and his loved ones die.

Okay, now my plan is going to kill some folks, and everything I cared about saving is dead anyway. Fuck em all. Why stop now? I was being nice before, but if they really want to see how ugly and petty their world is, that's great. Let's all take a bullet train to hell.

Evil doesn't happen all at once, at least in most contexts. (If you were full-blown psychotic or sociopathic out of the womb, however...)

I could suggest the seductive nature of power, that the more of it you have the easier it is to solve problems that way, until it becomes your default mode of operation, and the problems you're solving are very far from you personally, even far from actually being a reality yet.

I could speak of the nature of great tragedy, and the effort to undo or erase it, literally or metaphorically. Revenge is metaphorical and pretty easy to go for. But for all I know you could be dealing with a world where it's possible to literally undo the past, to return the dead to life, to rewrite history in the most literal possible sense. And damn the consequences to the order of things and damn the cosmological critters if they get in the way. If you have to kill the person responsible for allowing the dead to pass on from the world to keep your loved ones in it, what's a little deicide between friends?

Then there's good old Motive Decay. Maybe you cared enough once to start the quest, but now at the end it all those motives seem so dull, so silly, but you've got some new ones that could use a workout. Maybe staring this long at the abyss caused it to suck out your eyeballs, hop in your head, and start driving you around like a car, either literally as The Corruption taking hold, psychologically as having acquired an interesting set of mental issues along the way, or metaphorically as having reach a plateau of simply not giving a fuck.