at this point it is a crime to kill them, if you had killed them on the spot it would have not been a crime.
reasoning: you made them no longer an immediate threat to your life or your family, and no longer a threat to anything in your home(as they are disarmed, on their knees begging for their life), at this point you should be calling the cops to have them arrested... if they try to run... it is still a crime to kill them... now if they run then suddenly turn around back towards you, their life is forfeit)
and I live in Texas, home of shoot them intruders dead in home defense.