I'm not familiar with that show, but in real life you cannot take action into your own hand. Not because it's wrong killing say someone who abuses children, but because the law has to protect those until proven guilty. Many times people have taken the law into their own hands and killed innocent people. Then it's like oops.
Exactly. It's probably not against country's law to kill in self-defense... what really matter is that an innocent person is not killed... that is why having solid proof is very important. If the law is very effective on careless killing, people will probably go for the non-lethal approach, so that when you shoot a murderer in self-defense with very effective non-lethal weapon, you get the police to make arrest him/her(better on that spot).