The snake in the garden (or the devil before he did that, assuming you believe the biblical history) is a good one. The question is though.. if you killed the snake.. and it resulted in your birth/existance/everything you knew being non-existent, would you do it? The good part would be that the people that WERE alive would be in paradise. Bad part would be that you wouldn't be there to enjoy it
.
Of course, killing the snake/satan might not help. Some could argue that man is inherently (sp?) full of flaws, and that Adam and Eve would end up disobeying/angering God in some other way, causing us to be kicked out of paradise anyways, making your attempt all for null.
As for the Hitler stuff, I agree with most of the others.. who knows what would happen, who would take over? It's another situation where your own existence may be removed... a gamble you'd have to take if you think it's for the greater good.
I find it funny how no one has really said anything about their conscience. The fact that you're killing someone evil still means you're killing someone. May not be as easy as you think.
I've never killed anyone (good or evil), so I don't know