"They started hitting me first. So I hit back"

You:

First of all, I understand you think it’s okay to do, because you think it’s already being done by aggressors. But regardless of who started it, by saying it’s okay for self-defenders to do this you are also saying it’s okay for anyone else to do this. Because, if it’s a good thing to do, then it must be a good thing to do by someone you don’t like.

Your argument has no merit if this were an actual self-defense situation. Which Trump's followers believe it is. So you'd need to argue that it isn't a self-defense situation. Your argument wins no one over who needs to be won over.