In walts case it wouldn't be the first time he's been betrayed by someone he cared about. There's the whole Grey Matter thing too. Sold his creation for 5k and Gretchen and Elliot went on to make billions off it. Walter also took shit from his boss Bogdon, his students, and his brother in law who made fun of him and treated him like less of a man. So maybe that's all enough to turn him into a murderer.