Complete Idiot said:
So then it is interesting to think about the triggering mechanisms that happened during the show to send Walt over the edge to basically evil. I think he liked hearing how his product was superior to all others. I think he liked realizing how much money it generated. I think he liked the feeling of controlling someone like Tuco. I don't think he liked killing Krazy-8 per se but maybe liked realizing he had it in him? All came together to create a new Walt, relatively quickly. Short of a mental illness I've not seen life fundamentally change someone from good to evil in their actions, but then maybe this is the point where I am over analyzing it. Ultimately it's a show and a fake character, and it was done so well.
Other then making a product that ruins people's lives and blowing off his several opportunities to get out, much of the day to day evil that he did was necessary to save himself or Jesse. Watching Jane die was evil, but somewhat "necessary", since Jesse and Jane would have likely drugged themselves to death and eventually Walt would get exposed. If Walter had let Gus kill Jesse early on, things would have been much easier on Walter, but he couldn't let that happen. From that came Walt killing Gus's guys, having Jesse kill Gale, etc. Walt initially was going to escape with his family, but since Skyler spent their money, he had to stay and kill Gus. Even killing those guys in the prisons was "necessary" to save himself. He couldn't just pick up and leave his family like Mike. One his senseless killings was Walt killing Mike. But think about it, Mike himself was a monster. He killed people when ordered to without conscience. And even then, Walter felt bad and apologized before Mike told him to shut the F up. Much of his evil was intentionally morally ambiguous.
Another thing I find interesting, is that any of us probably would have been able to predict that Walt's actions would have destroyed his family and cost countless lives. But did any of us call him an A-hole after episode 1? No. We were cheering him on and hoping he would succeed. We celebrated when he killed Crazy 8 and Tuco. It wasn't until after he had destroyed his family that most of us say, "what a jackass for putting his family through that".
For me, the moment that I realized that Walt was truly evil was not when he had killed anybody, but when he was whistling after him and Jesse discussed that kid on the motorcycle getting killed. It didn't phase him at all.
Anyway, that show kicked ass.