It's really not clear why he's a drunken abuser. It's clear that the mother is an emotional abuser and manipulator (I think there's enough objective evidence to conclude that that's not all in the narrator's head), and that the narrator vastly prefers the father although she sees some of his faults (and his other faults are clear to the reader despite the narrator's passing over them). Perhaps his faults are caused by the situation he's in, or perhaps his faults lead him into that situation - the text doesn't take a stand on that.
no subject