My thoughts are liability always exists, because even if you do get past the legal system with a clean slate, you can still be sued by the perp if he's still alive, or a member of his family.while the liability certainly exists, I don't think it to be a major concern of citizens carrying revolvers for defense, the way it is for police departments. The primary reason is that police FREQUENTLY hold suspects at gun point, while armed citizens do it much, much less often.
For one thing, some ambulance chasing lawyer will be looking to make a buck, by saying you were willfully negligent and with video cameras everywhere, not to mention a live witness who could testify that you cocked the hammer, before taking out their beautiful innocent loved one. You could still end up losing in a civil case. And that sorry lawyer will lie in court and accuse of cocking the hammer and then accidently firing the weapon. It has happened in the past.