There would no reason for him to go after them. Not saying its right but maybe he thought he needed protection. After all his daughter was murdered and not by a stranger.
Why would he need protection? Why would someone threaten the person who found HD's body? Why would someone threaten people around his other daughter? This is unbelievably weird.
This does not make sense, are you trying to suggest that he is the real target in all of this?
What sort of thought process would go into "Oh, I killed HD, so now I must kill CD too"? That just does not compute, unless there was something else going on and HD's death was just collateral damage.