I've read a comic book where the "mastermind" of the story did exactly that. This was in an alternative universe where World War II lasted longer than in our world. The mastermind became chief of the third Reich (Hitler was dead at that point, I don't remember how), devised a plan to take out all the industrial world with a biological weapon. He also chose a select few to be sheltered in bunkers while the world healed itself, as seeds to a better world.
The thing is, he despised the Nazis. Yet not only he took their methods, he amplified them: he judged the current world rotten, and set out to cleanse it. The twist is, he ensured he himself died in the process, because even as he saw no third alternative, the sin was too to great to go unpunished.
I must say that I like this reasoning, because it gets rid of a great deal of rationalizations typically made by those in power. If your cause is worth killing millions of people, but you're not among them, then your reasoning is most certainly flawed.
Edit :Excellent suggestions in the comments. Two of them stood out for me:
We often use "insane" to describe people whose behaviour or beliefs are below the sanity waterline. But as most must would agree here, you cannot call someone insane with a straight face just because he happens to believe in magic.
I'm currently watching Future by Design, a documentary featuring Jacque Fresco and the Venus Project. Jacque came up with this word, "unsane", to describe people who basically, aren't rational because they haven't been exposed to the right ideas yet. Which would be different from "insane", which is more about irrevocably irrational people.
I like this word, because there isn't the tone of accusation we find in "insane". This neutrality makes it easier to say that we can do something about it. Insanity should be eradicated like vermin. Unsanty on the other hand can be fixed.
So, do you think this word, "Unsanity" might be worth using?