Like the others have said, being hit once, at a time when it was acceptable to hit your kids, doesn't mean your mum was a terrible mum. As for being scared of her, well, that rather depends on whether you were thinking "I hope she doesn't find out that I've lost my coat or she'll KILL ME" or, conversely, cowering in a cupboard hiding from her when she was in a drunken rage.
The psychotherapist Albert Ellis used to say that if you could pinpoint "what had made you screwed up" it wouldn't necessarily help you. It might just "make you MORE crazy!" because, if you blamed your mother for ruining your life due to her momentary flash of anger, you could end up developing a massive sense of unjustified resentment against her which might destroy an otherwise reasonable relationship.
I don't think he's quite right. Some people do have terrible experiences which shape their view of the world and themselves and which it can be a good thing to understand. But that doesn't mean to say that if you feel terrible it MUST be because something terrible happened to you. "Men are troubled not by things but by their opinions of things."
Mental illness - and I speak from experience - very often has an obsessive element to it. Thoughts run round in circles in your head. You wake in the middle of the night in agonies of guilt because of something mean you did at primary school. You relive, endlessly, the shame of having the piss taken out of you for wearing the wrong shoes.
Now if something really dreadful had happened to your brother you would want to help him put it behind him and move on, wouldn't you? You would want to tell him that he doesn't have to live as a victim for the rest of his life - that he didn't have to feel shamed or sullied for what happened to him, and that he was in no way responsible for it.
But in that hypothetical scenario we'd all appreciate that putting really traumatic events behind us is bloody difficult. Here, though, what happened wasn't, I think, all that awful. Upsetting, perhaps. But isn't it even more important that a minor event shouldn't have to cast a shadow over him?
Sounds like the best thing you can do for your brother would be to say "But, you know, Mum didn't mean it. She was just cross and worried. And everyone used to hit their kids then. Doesn't mean it's right, but it's not that much of a big deal." (I don't know what your relationship with your mum is like now. It'd be good if you were able to say "But she still loved you.")
If he's "very" unwell then he's hopefully getting the help he needs. xx