The legal problem occurs when the suspect is no longer an immediate threat.
Like it or not, once they are out of the house and running, you are not legally justified in plugging them.
This statement: once they are out of the house and running is no longer an immediate threat, is a falsehood. Just because the perpetrators were outside doesn’t mean the victim was safe. His house offers very little protection against gunfire. It can’t be considered a safe haven. If the person running still has a gun in their possession they are a threat. They could turn and fire in a fraction of a second. They could be running to obtain cover achieving a stronger tactical position to return fire. In fact how would a person know if they did or did not have other more powerful weapons like rifles stashed they intended to retrieve with the intent to fire back? The fact is his immediate threat was not over until his attackers were disarmed or dispatched. That is what the jury will have to consider and decide how those factors will be applied to a verdict.