Why Kill Hunsakers Daughter?

It made no sense. He knows everything about their operation, is threatening to go to the police and instead of just killing him and making it look like an accident or a robbery gone wrong, they kill his daughter (thus pushing him further over the edge against them) and leave him alive? I could see threatening her safety to keep him quiet if they didn't feel killing him was the best choice, but it just seems they needlessly made their situation worse.