but these men started the felony, the civilian simply ended it.
Yes these men did enter a house, breaking a window or a door and destroying some property. He ended it by killing them. If you cannot see why this is wrong then I do not understand you.
There is a nice word in German law called Verhältnismässigkeit. I suppose something similar exists in US law. Proportionality perhaps?
If you break into a chewing gum dispenser you do not get put into prison, if you break into a house you are not shot.
This is different if someone protects himself and you can argue that it is ok for home owners to shoot burglars in defense because he cannot know if they are armed. It is NEVER ok for someone to shoot fleeing persons when they are no harm to him or someone else. Because of this we have a "Rechtsstaat" that means the government has the monopoly of prosecuting people and using force to uphold the law. Self-Defense is an exception. Executing people like this should be prosecuted.