Oh, that goes without saying. Everyone always thinks they are acting for the best reasons, for the most desireable outcome. Otherwise they wouldn't do what they are doing.

If Hitler had won and subjugated Europe, all of Europe's history books would be written from the angle of the winning side, and praise the philosophy and moral, intellectual, and physical superiority that drove the Nazis to victory over the lasser races, and approve the wisdom of the ethnic cleansing that made life in Europe so much cleaner, safer, healthier, more prosperous, and more beautiful. And after the first generation who grew up reading those history books were adults, everyone would just automatically "know" that it was true, obviously, because life was good now.

As far as crazed lone gunmen who burst into offices and murder officials... of course they have their reasons. Those reasons are clear and strong enough to motivate them to rush out with a gun and shoot someone, or many people. We can't usually make good sense out of those reasons because the minds of the shooters are troubled and confused, but they themselves are certain they are doing what must be done.

More research needs to be done on this, and captured killers studied, so that we can learn to recognize the signs of overwhelming obsession and impending rampage before it happens, and educate the public to protect itself.