While a gun is useful in certain situations, I don't think it would've protected him in this instance, where he was sucker punched to the point of knocking him down and unconscious.
If he had used a gun in self-defense, there would not have been massive legal costs, jail time, or problems with Al Sharpton. It's only when the situation is murky that those things come into play. Like the Trayvon Martin, when it was the guy who used a gun in self-defense who was the instigator of the incident in the first place. That's not as straightforward as this instance. This sort of thing happens a lot in our country. No criminal charges are filed, no racist claims are made. Open and shut case. The man who sucker punched and killed the other man will be charged with a felony.