Well, I would caution you that shooting someone who doesn't have a weapon, regardless of that person's intentions, is a risky decision and will likely lead to your arrest. The argument could be made that you are looking to pay him back for the beating he gave you.
Plus, if you had your firearm on you then and he knocked you out, you would have armed the bad guy.
I'm an advocate of OC, but there are two very important points you must consider:
1. Be aware of your surroundings. You must be aware of everyone and everything going on around you. Many criminals have guns, but you don't want to be the one to put a gun in the hands of an unarmed one or worse, have your own gun used against you.
2. REMOVE YOURSELF from any potential dangerous situations. A gun is a tool of last resort, not a badge of invincibility. If you carry a gun, you have to check your pride at the door and take yourself out of situations before the need arises to use the gun.
If you're going to carry and this guy comes around, leave the situation immediately. It's ok to be "scared" of him and all the other things he'll bark at you as you leave. On the other hand, if you are unable to get away and/or this guy pulls a weapon on you, you must do what you feel is necessary to defend yourself and others. Just make sure you have hollow point ammo to minimize risk to innocents.