I'm Dr. Jackie, an interpersonal communication researcher/professor and relationship expert. And I am so sorry for you for what happened.
Let me start with, "It's NEVER EVER EVER OK to hit someone else unless of course, it is to defend yourself or defend say a child." Hitting is unacceptable. The fact that he hit you over a comment you made and certainly not in self-defense makes me wonder what else he may have done to his own girlfriend. While I don't know him and certainly cannot say for sure because I don't know him, I can say what research studies show. And what they show is that if this man physically struck you over words, I would NOT BE SURPRISED in the least if he hits his girlfriend. Again, please understand I am not accusing him at all and never would unless I had facts. But his behavior toward you is highly, highly suspect. Again, it's very likely he abuses his girlfriend--a lot of men know "how to hit a woman" so that bruises and such cannot be easily detected. According to statistics, if he is abusing women, it will only escalate unless there is intervention (i.e., counseling). Then it takes working through anger issues, sometimes trusting in a Higher Power, etc.
Second, I don't know what his defense is, and quite frankly, I don't care. There IS no excuse OR justification for hitting, again unless it's self-defense or defense of a helpless child/animal, etc. I don't know what else he would possibly want to talk about? It sounds to me that it might be more "excuses." I would listen to this "talk" only if you want to. But as for trusting him, no I wouldn't. You can forgive him for what he did. In fact, I would encourage that, more for you than for him. In other words, you have probably seen this in the media when a murderer is confronted by a family member of the person he killed and the family member extends forgiveness. It's more about the family member getting peace and feeling better inside and letting go of the hurt and anger. Otherwise, the rest of THAT person's life would be filled with bitterness.
Lastly, I know you have known him for 9 years. But HOW WELL have you known him? Again, statistically the research would peg him as an abuser. Now, there is an exception. It's rare, but he may have developed a tumor of some sort in his brain blocking certain impulse control mechanisms in the frontal lobe. I have read some medical articles on this. Other possibilities if he has never hit before and this is the first time could be attributed to some type of drug use like steroids, which can make people very, very aggressive. But unless the meds are doctor-prescribed, he is still responsible for his own behaviors.
I hope this helps. I am so sorry this happened. Remember, forgiveness is more for you. I would definitely try to forgive him. But as far as trust? That is something you will have to decide for yourself if you choose to interact with him in the future.
If you want additional information/research citations, please let me know. I definitely want you to be satisfied with my answer.
Take good care,