I tgot the impression that Alison's father was around and from what she said to Catherine, Daryl had picked up that he was the product of an incestuous rape. He also had special needs, I think, and was probably abused at home by his grand/father and was certainly bullied and made fun of at school. I think Alison kept him at home after her father's death so that she could protect him as much as possible.
It is possible - maybe even likely - that he saw/heard his mother being raped repeatedly, and heard her being threatened and abused, and that this fed into his own twisted idea of what men and women did in bed, and this may have become further corrupted by pornography he encountered online etc. He probably took out all of his frustrations from being bullied and mocked by other children, and then other adult men, on the women he assaulted.
He certainly knew that what he was doing was wrong, but probably couldn't realise why.