His motivation was to steal Chris's truck.
He really liked Chris's truck and the first thing he does after killing him is to steal the truck and take it over to his sister's and other people to show it off.
When a scum bag kills someone for their car, which happens every year, nobody says "oh that poor guy must have mental issues." So, just because this guy wore a uniform in a past life doesn't mean he's excused from killing Chris due to PTSD. PTSD doesn't make you a sociopathic killer who likes stealing trucks.
Nothing flipped him out. He didn't have some mental incident or Chris would have seen that coming. The dude waited for an opportune moment when he could shoot two men and steal a truck. That simple.
Scum bag should get lethal injection, but the prosecutor isn't pursuing that.