I guess maybe the answer is, why not?
I cannot get past the “why” and I am really, really stuck here. Why did he start getting drunk every day? Why was he so abusive? Why couldn’t he get help before things ended the way they did?
Alcoholism is a force, a drive to drink a call from the brain. Think of it like being hungry. If you are truly hungry think about how hard that is to deny. Don't eat for today and see how you are around 11 tonight. Chances are you won't be thinking about anything other than the eggs and toast in the kitchen, how good it is, how much better you would feel if you ate it. If you ate it you wouldn't have to be thinking about it anymore, your body would just be happy you took care of that.
Well, that's probably something akin to the desire for alcohol. That's why he drinks, nothing to do with you at all. You left, you have zero contact with him and still he drinks.
Now of course you have to eat and he doesn't HAVE to drink, he could go to AA or some other place for help - or he could just drink a beer. The difference with alcohol is, of course, it serves other purposes, it blocks out thoughts that are unwanted, feelings that can't be dealt with.
That is a huge force. Why couldn't he get help and have a relationship with you? Well he does not have his rage or drinking under control is the answer to that. You see it as a choice, I doubt he sees it as a choice at all. Also keep in mind that if he went out today and got sober and got help it would be years before he is in any kind of mental space to have a relationship. None of the healing comes overnight and you don't even actually know him as a "sober" person.
My brain can’t comprehend that one person would do the things that he did without some drastic reason that compelled him or her to behave this way. It’s just completely outrageous. I would feel better if he would give a rational explanation—although I don’t believe him capable of rational thought, much less a rational explanation.
From what you have written here you seem to be taking this all personally. So I think what you are really asking is why did he do this to you? What did you do to deserve this?
Well actually, that's probably where your thinking is in a loop. It's not actually about you at all.
Do you believe that when he was sitting in the dark, watching TV, about to have his next outburst that this had anything to do with you? Rage is an inside thing, not an external thing, it's not rational. Have you ever felt rage? If not maybe that concept is not easy to grasp.
Do you think when that force is telling him to drink that you crossed in to the equation? How long could you go without eating for him?