You’re right. There’s no point in confronting him any more. Although I can’t fairly diagnose someone on the basis of a letter, from what you describe, your father is a very, very sick man. You aren’t going to make him take responsibility now. He never did.
If his behavior had been reported to authorities when you were young, there would have been consequences. In many states, he would have been separated from the family. In most states, his treatment of your brother would have been considered attempted murder. I’m only sorry that no one knew enough about what was going on to put a stop to it. You and your siblings deserved protection. My guess is that your mother was as terrified of him as you were so she couldn’t help you either.
All you can do at this point is to refuse to stick around whenever your father allows his cruel side to come out. You can’t make him take responsibility but you can inform him that you aren’t going to put up with it any more. At the first inkling of this stuff, simply leave. There’s no point in talking. Actions speak much louder than words in situations like this. If he still persists, then you may well decide that there’s no point in being around him.
I hope you can find some older male friends to give you better role models for what it means to be a decent man and a loving father. As a young man, you need to find ways to be a real man without adopting any of the brutality of your dad. It’s really fine to go looking for people to fill that role in your life as long as there is reciprocity of some kind on your part.
I wish you well.