"That's pretty much what you seem to be saying."
No, that's not what anyone is saying. There are two independent facts: All three of the people Kyle shot had criminal records, some pretty nasty. Rosenbaum, for instance, was a repeat child rapist. We have testimony and/or video evidence in all three cases that Rosenbaum, Huber, and Grosskreutz were attacking Kyle before he shot. This is THE DEFINITION of self defense.
"Didn't one of the people chasing him have a handgun? If they really wanted to shoot at him they would have done so."
Yes, Grosskreutz had a gun. He was getting ready to shoot Kyle when Kyle shot him first. We even have his word after the fact that he wanted to kill Kyle. Gun's aren't magic. They take time to aim. Kyle was faster.