I don’t think it’s an overreaction, though on the other hand, abuse can be isolating and make people afraid to speak out as well. You never know what will happen if you confide in a colleague, especially a new one, and sometimes even an abusive boss can seem/literally be better than not having a job (I don’t condone working for such people, but it’s easy to see why someone might stick with a terrible job instead of leaving). It’s hard for me to see why he didn’t do the work he was meant to while covering, though.
I am sorry you’ve had to deal with all this, whether he’s out for himself or not it sucks, either way. It probably would be a good idea to maintain a more professional boundary with him from now on, if you feel he’s not entirely trustworthy.