I don't think you need to apologize...
1. I don't blame the dad for his son's actions.
2. OK, he's greiving the loss of his son (a mass murderer). Murderer aside, it's ok for him to grieve, I can't take that away from him. But just because his son died does not give him any excuse to be an idiot and make ridiculous accusations and say stupid stuff. If he doesn't like the US, he's welcome to leave.