If they were muslims why where they opening Christmas presents?
Maybe that's what the killer objected to, the fact that they were celebrating Christmas.
They were obviously westernized. My baby sister's Muslim husband just attended a family Christmas get-together at my dad's house in San Jose. He partied right along with everyone else.
Still, you never know when 'sudden jihad syndrome' is going to take one of these people. There have been lots and lots of cases of westernized Muslims, who suddenly 'went jihad', to the surprise of everyone who knew them.
There are no absolutes in human nature. These Muslims had lived in the US for a very long time, and had adopted many (if not most) of our Western customs. Doesn't mean that they had also abandoned all of their Muslim religious customs.
There's no doubt that the father was disturbed about his daughter dating a non-Muslim man, and being on the verge of striking out on her own. Somewhere between the pressures of his business collapsing, losing his home, separating from his wife, and being twisted in knots about his daughter's social life, the guy snapped.