Hi all, very long time lurker on both AIBU and the Relationships boards, but I just created an accountant to open this thread. I’d like to hear external opinions to check if I am overreacting or not.
I have been on 4 dates with a man who has been separated from his ex wife for over a year, now in the process of divorcing. No kids. He is a nice guy and I enjoy spending time with him. We have never discussed our relationship status. For context, I live abroad and here we are not in lockdown.
Yesterday I had dinner with guy (date number 4), and he told me that a few days ago he sent a letter (handwritten) to his ex wife to announce to her that he has now moved on as he is with me. What the hell?!? 
He also added that apparently she had a breakdown upon receiving the letter, as a mutual friends of theirs told him. He looked really satisfied about it. I asked him why he was happy his ex had reacted that way, and he said that he is happy to have had that further closure and that he has moved on. He says he sent the letter because he didn’t want her to find out that he is seeing someone from someone else 
AIBU to think this is a gigantic red flag and that 1) he must have a lot of unresolved feelings for her to have done such a thing, 2) he does not come across as a nice man if he is so happy he has caused pain to his ex?
I am considering ending things as a result of what happened. Am I overreacting? It made me feel really uncomfortable.