There is no understanding it. There is only acceptance that he thinks there is not a problem with him and his drinking. He cannot accept that there is a problem with himself, so he acts like there is a problem with you. Deflecting is very powerful.
I can remember, when I first left my XABF, I told him that I was staying with my parents for the weekend. I told him that I was going out to dinner with them and then spending the night for the weekend. His exact words to me were, "are they taking you out to celebrate you leaving me?". I was disgusted. The real reason I was with them was because I was devasted. I needed company and support. They are always willing to help me when he was not. Since he still couldn't blame me, he then blamed my parents.
I have not been able to come to grips with the anger that my XABF has for me. He has said many many mean things in text message that show not only anger, but cruelty. He once told me that he could never be with me again because he knew I would leave him. But he would be willing to let me use him for sex. WHATEVER!
There is so much more to come for you. Sometimes being "cordial" doesn't work. Sometimes complete detatchment is the only thing that will help you keep your sanity. I would suggest this in your case, if you are ready for it.