I was beaten by my boyfriend. After I packed my things and left, I sent a message on his QQ saying that he had beaten his girlfriend. Is this behavior excessive and irrational?

You'd better ignore such people and leave as soon as possible. Don't let him have an accident because he doesn't want to be wronged and has nowhere to vent. Just leave him at once, it's nothing to do with him. As for who he is and what his friends think of him, no matter how he is not human or how he harms others, it will not affect you. This is what you have to do. Although your behavior is reasonable, it is really unnecessary in the long run. Without saying anything, leaving quickly is the best result.