children who grow up around domestic violence, will sometimes internalise hatred of the mother for not protecting them/herself. Or they will psychologically relate with whoever is doing the abusing, and distance themselves from the abused because they have no other way of coping, which is why these things can be so cyclical.
in this case, the family sounds truly ducked up. I don’t think there’s any way of rationalising what he thinks/does, because the causes aren’t rational. I haven’t read upthread but he should not be out in normal society. He needs life long, secure, psychiatric monitoring.