Killing an enemy who no longer poses a threat (i.e. is incapable of mounting resistance) is illegal and wrong, both in the civilian world and in combat.
Now, many times what looks like a disabled enemy on video is not the case. Many combatants keep fighting after being shot. Suicide grenades, hidden weapons, etc. pose a serious threat to those trying to detain prisoners. Also, there are many cases where a fight continues until one side is dead/mortally injured. It is usually advised to continue fighting until the threat has OBVIOUSLY been ended.