What makes you actually think that people would want to go around killing others?
Because people already do. If you got rid of all laws and punishments what do you think happens? It would be literal anarchy.
Couldn't they at least be deterred by the fact that others could kill them? That is a good question though. There is a lot of talk about libertarianism on this forum. I never quite thought out completely the police bit. I wonder what would happen with crime in a libertarian society.
It would be kill or be killed. Like the wild wild west (wikki wikki wah wah).
Cannabis sounds like a great business idea. Maybe you could just move to a country where it's legal and get on that. 
Sure, let me just leave all my friends and family to go to a foreign country just to start a business. This isn't something I'm desperate to do but would do if in my own country if it was ever legaslized, but I'd like to stay in my home country right now thnx.