It's not as simple as just anger. I think that people that are in highly stressful situations for long times suffer from permanent brain damage as a result, that may manifest itself slowly over time, or a switch might get flipped all at once.
I think there is something in Holmes' past that might explain all this. In the case of Charles Whitman, an overbearing father who demanded perfection, and Whitman's inability to satisfy his fathers standards, are imo what caused him to snap. Everything about what he said and did leading up to his shootings at the University of Texas at Austin suggests the same. Workplace shootings too, hating the job but needing to keep it, putting up with crap and worrying about getting fired can make a person severly stressed. Then get fired...it can set some people off.