How hard do you have to punch someone to kill them?
Maybe you should learn to throw a good punch then worry about where to land it. If you can hit hard enough that it hurts almost anywhere then it could make most assholes back off. Try getting some experience before you make it too complicated.
If you throw someone they could break their neck or hit their head and die. Fighting is risky and dangerous, and you really have to be able to hit fucking hard to kill someone by punching them(or sit there whaling on them forever). If you're grappling someone his friends could beat you while you're trying to fight him, it puts you in a vulnerable position against unanticipated 3rd parties.
To sum up, hit the heavy bag, hit the speed bag and hit the weights hard and heavy(to get strong). Dont neglect wrist or neck strength.