Cruz had been threatening to kill everyone for at least two years prior to the shooting; neither she nor anyone else should have any obligation to put up with that shit.

Ah yes, because when someone is deeply dangerous and you are afraid of him you should....make the situation worse by feeding into his victim complex and hatred of you by actively isolating him.

So I'll say it again:

Because there is a far gulf between "we didn't know him" or "he was kinda weird so we kept our distance" and "we outright ostracized him." You see how one is passive and the other active? That's where her responsibility lies.

She fucked around, and everyone found out for it.