Having a hard life/abusive childhood etc. is one of the classic lines abusers use all the time to get their partner to put up with it and give them more chances. It's listed in Lundy Bancroft's "Why does he do that" book, and I've heard real life examples of it being used as well.
Lundy Bancroft points out, correctly, in the book that if someone has been a victim of abuse/had a hard or bad childhood etc. - why would they want someone else to feel the same way that they did. Why would they want to do that to someone else. And why wouldn't absolutely everyone who's been abused act like that .. they don't.
It's the ultimate "feel sorry for me", "take my excuses", "give me more chances", "you'll be treating me badly and abandoning me too if you leave" manipulation. If you'd really had a hard life, why would you want someone you supposedly care about to have one too.
Bereavement - so does everyone who is bereaved hit their partners too? My theory is that ppl who are capable or hitting their partner might do, and those who don't, don't. I'm sure there are a hundred mumsnetters on here who could confirm their partner never it them during/because of a bereavement.
At the very least he needs to admit to what he did, agree there are no excuses, take responsibility for it, agree to counselling/a programme, do it and not repeat anything similar.
I worry that other posters are right in saying he did it because he feels entitled to - in the same vein as apparently thinking he's entitled to throw all the work with the baby onto you.
Do you know anything about his past relationships or could you find out - I wonder if he has been violent before.