Using deadly force to stop a threat either in combat or in civilian life is necessary in many cases. Once the threat is stopped going further and killing someone based on their future potential to threaten again is tantamount to murder.
Honorable combat has limits as detailed in the UCMC and Geneva Conventions. Civilian self-defense is just that - defense.
So, no. Killing after the immediate threat which necessitated using deadly force is no longer present is not acceptable. Doing so is unlawful and immoral vigilante behaviour - judge, jury and executioner is not anyone's right to excersise.