I think he's just plain crazy.
And, as was the case with Major Hasan, everyone knew he was crazy and and a danger to himself and to others, but rampant political correctness prevented anyone from even thinking about doing anything about it.
Yeah, I suspect that in his mind the grievances are legit. That is true with most crazy people, no matter how absurd the position may be to a sane person.
I find that, though I may find a person’s actions abhorrent, I also find myself, immediately on hearing of things like this, wondering if their complaint is legitimate, even though it does not justify their actions.
I remember the first thing I thought when I heard about McVeigh bombing the BATF headquarters on the anniversary of Waco: Well, at least the guy picked a military target. I didn’t side with him, but my reaction was different than it would have been if he had blown up a shopping mall.