The "Common" law that is prevalent throughout the world for the most part and is understood is basically this" Do no Harm to others nor cause another loss". Even if there were no written laws, that common one would still be enforced. For those people who think that if somehow the human mind was not repelled by killing others that we would have people getting killed all the time. Hate to tell you, but the Human Phsyche does not work that way. You have no idea how much intestinal fortitude it takes to kill another human being. Its not like flipping a light switch or something. Kill someone and it will haunt you for the rest of your life, whether it was justified makes no difference to ones mind. Knowing that you have taken a life and taken something away from everyone who was associated with that life will never feel right in your heart.
There are people who are mentally screwed up who would kill people whether it was illegal or not. They still do, ever wonder why serial killers are not scared of laws against murder?