I think a HUGE part of the reason men are killing their pregnant wives/girlfriends is because they don't want the responsibility, either financially or emotionally, of being tied to a family.This is most likely the case.
But, if a man is against having a family so strongly that he'll kill his wife/girfriend and unborn child, why didn't he bother wearing protection? Is double-homicide much simpler than putting on a condom?
It doesn't make sense.