Since communism is meant to be a stateless society, who would be doing the whole murder part?
You sound like one of those people who think that the socioeconomic system that existed in, say, USSR is communism. It wasn't, and not even Soviets themselves claimed that it was (that's why it was called Union of Soviet Socialist Republics, not Communist Republics).
what you do with people in your ideal communist society who want to put in above-average effort
For starters, communism is not about "average effort". It's about what effort every particular person can and is willing to put in - "from everyone according to their ability".
and reap the extra rewards
"To everyone according to their needs".
What happens if you claim more than what your needs are, attempting to justify it by increased contribution? Well, communism is only possible, even theoretically, in a society where the vast majority embrace the underlying notions. So what you'd get in that case is them simply not sharing with you. In a society that does not recognize private property, even without the use of force, you'd basically only be able to claim that which you possess at any given moment - anything beyond that would be beyond your claim, as there is no state to enforce your property rights against other people's claims (that's what many ancaps get completely wrong - they think that the abstraction that is private property beyond personal possessions can exist without a state to back it).