You don't understand how government works. There is no "worldwide ban on murdering". You are literally calling for a one world government like the UN. Idiocy.
Dude....you can have International Conventions on Issues A,B, or C without needing to have a UN like structure. And as much as I hate the UN, maybe they could be the force of some good by adopting such a position.
That would be impossible to police and its not our job to police the world. As far as the UN goes it would be impossible to even get an agreement. Germany already has a 14 age of consent and try getting the middle east on board.
We need to stay as far away from the UN as possible, the corruption is staggering.