Hate speech does not give justification to physical violence. I am not saying you said that, but I want to make it clear for the next point.
Physical violence runs the risk of killing the other person. All it takes is 1 hit to cause some people to die. If you start physically attacking someone you are very much putting their life at risk. If you are going to put life at risk, then you run the risk of someone putting yours at risk as well. So if the argument is one of equal force, then it gets very grey because throwing punches runs the risk of killing someone and words to not.
People can claim baiting all day, and it does not justify physical violence. Provoking people is not a great idea, but it also not illegal and for good damn reason. Me living with my girlfriend when we are not married is considered deeply offensive to some. My actions in no way justify the offended people to do anything other than bitch and complain. I also don't have the right to do anything to the offended people other than to use free speech.
Just because this guy said a bigoted thing does not justify physical violence. If you are going to use physical violence and thus put this man at risk, he has a right to defend himself. If people want to say its unequal, then fine, but lets admit that the first jump was done by the person attacking the guy in the truck. Like I said before, people die from minor physical altercations.
All this is predicated on what I know thus far. If the person shot had not thrown a punch, then its murder. If he had, its self defense.