After witnessing an execution, my first instinct would be to eliminate the threat.
Every situation is unique however. While in this situation the circumstances make the killer's actions fairly clear in many they are not.
The last thing you want to do is kill someone using justified lethal force before understanding what is really happening.
Nor are you judge and jury for criminal actions if an actual criminal is no longer a threat.
In fact in a large number of public self defense cases especialy involving firearms the defender is initialy reported as an aggressor by the majority of witnesses.
The reason is clear. The first thing that caused most people to even notice something was going on was the sound of gunshots. They then look and see someone harming another person. At a point when hopefully things are working out best for the armed defender.
Thier mind often jumps to various conclusions. Such as the conclusion they are witnessing a murder.
Afterwards when answering questions and giving a statement thier mind has began to exxagerate the clarity of what they saw. They often become positive they witnessed a murder, they saw the cruel killer standing thier discharging the weapon as the victim collapsed. When in reality the "victim" could have been a robber who failed in his latest attempt at predatory behavior.
So even with dozens or hundreds of witnesses sure they saw a murder, the guy could just have been a victim legaly defending himself.
The mind can make further assumptions based on appearances. If the shooter resmebles what they think bad people look like.
By believing they witnessed a bad person doing a bad thing thier mind also will seek to find other bad qualities of the individual. Like how they were suspicious before the event.
The valid defender leaving the danger to call police for example remembered as "murdering and then fleeing the scene". In some self defense situations, especialy when a group of individuals is involved like a group of young men or teenagers or gang members leaving the immediate scene to avoid further conflict with the friends of the shot individual can be in your best interest.
If you just defended yourself from a gang member in that gang's part of town standing around waiting for police can make you ripe for retaliation. Proper procedure is not going to protect you from "revenge" from thier homeboy.
If you just defended yourself from a violent individual standing around thier angry friends and family could also pose a serious risk.
How many parents do you hear call thier vicious child an angel after they commit a long string of crimes?
How many friends, girlfriends, or wives think a man that savagely attacks people over minor insults was a nice guy and are going to view the defender as over reacting?
The majority of situations are far less ideal than the perfectly clear and justified scenarios gun rights groups point out. In fact they point out those situations because the details make them so transparent to readers.
Consider police responding to a home invasion assuming the person firing shots is the criminal and needs to be dealt with. It could be you defending yourself from the criminal with your own weapon. Should the responders rush to judgement? It could also be you or family being executed by the invading criminal. Should they rush to judgement?
You hear an active shooting in progress. A madman is randomly shooting things outside. You look out and see an armed man with his weapon taking aim at someone else. Do you know he is the madman? Is he an undercover? Is he a good armed citizen who witnessed more of the situation than you?
The "innocent and cowering" target he is aiming at could be the active shooter kneeling down and reloading his weapon before he is going to continue his rampage.
The Tyler Texas court shooting had all those elements present, including the CCW holder that shot and hit the criminal twice with his pistol. So did the North Hollywood shootout with many armed undercovers running around. Even police and citizens in the area were reporting the armed undercovers running around as potentialy more suspects.
You need to assess the situation and understand what you are seeing.
Be careful of assumptions of what you see.
