The idea that this was the result of a "heat of the moment" reaction to some argument at a Christmas party is just absurd.
Heat of the moment is not driving back home, gearing up, changing clothes, putting on a mask, having the wife do the same, having the kid at Grandma's beforehand, then driving back, and calmly trying to massacre everyone who was there.
It's as absurd as insisting on calling the shooting at Fort Hood "workplace violence".