That's a lot of accusation to heap on the parents with basically no evidence whatsoever.Huge parental failure. But I'm thinking it was in the parents modus operendi so the child was being abused long before the abuser abused her. At least they stepped up at the end to stop the latest abuse but there should be some major lifestyle changes.
As to shooting the guy on the spot, I would not have had a problem with it, but it's quite possible the dad didn't want to spatter the guy's brains all over the room in front of his already traumatized daughter.