Honestly, I think some people in this thread are overreacting.

It's not clear the actual severity and frequency of the problem. You describe one recent time when he punched and slapped you and then say there have been other times when it was far less severe. How many other times? I mean if this is something that happens once a week then I would agree with the other posters that he is just unstable and you should leave him. On the other hand if it's once or twice a year and usually not nearly as bad as the one time you described and the relationship is otherwise good, it's probably something that he can learn to fix. Maybe through some anger management therapy or his own force of will. Ultimately it's your choice, though. If you feel you need to leave then leave.