Always the #1 reason why even people who are somewhat balanced will lose it- divorce, adultery, child custody. And this guy wasn't, with previous domestic assault charges but the Great State of Texas thought his 2nd Amendment Rights were more important.
No way to know but you could also wonder if online stuff didn't contribute as there are substantial communities that tend to run parallel or with the alt-right crowd where men are encouraged to wallow in misogyny and self pity.
#2 is economic- either feeling cheated or the workplace thing- a common mass shooting scenario.
#3 is your basic property dispute- fence line wars with neighbors and such, like what Sen. Paul was likely involved in.
Re: "domestic situation"
Allen, nox was talking about the reality of the shooter having anger management issues and a domestic dispute with his mother-in-law.
He was dealing with reality while you come along with a remark that had nothing to do with the reality of the situation.
that leaves you firmly in the position of belong a troll, not nox.