He was not using deadly force in response to a felony. He responded to a felony to stop the crime in progress and then used deadly force in self defense. Each action was legitimate at the time it was taken. I'm glad the thug is dead, glad I was not the one who had to shoot him, and disgusted with the criminal, the mother, and the parasitic prosecutor. Had he shot the criminal down from a distance and while there was no personal threat, that would have been what you are describing. What actually happened though was clearly both legal and neighborly of the armed citizen.
There would have been no personal threat if he had stood on the steps and dialed the police on his cell phone.