Honestly, given what the story says, I can't blame the father.

Putting myself in what I think were the father's shoes, I probably would have done the same thing. You walk in on your son being raped, as far as you can tell, would you not become enraged and filled with a blood-lust? If I thought my son or daughter was being raped, I would certainly act that way. Now, it would be wrong to deny that the father almost certainly created the environment that would make it appear to be rape, but if we're going up the chain of actual cause, then we blame the society that created the father who necessitated that the son's consent remain secret and unforeseeable. These things shouldn't be ignored, but to place myself in what appears to be the father's shoes, what he did does not seem wrong to me. He reacted to the situation as most parents, even pedophile parents, would.

Did he know of their relationship beforehand? I see no evidence of such. Did he know they had sex multiple times before then? I see no evidence of such. They may have had sex multiple times, but this is when they were caught. I can't fault the father for reacting to what appeared to be a rape of his son, given what I know of the situation. Was it the best response? Maybe not, but I cannot outright condemn it with what I know. I don't deny that he took it too far, but I know the kind of anger that I imagine he felt. To become so angry that the only thing you can focus on, or even comprehend, is causing pain to the other person. It's incredible how powerful such an anger can be.