Because it's the right thing to do today. I don't not kill for the reward of an afterlife, I just think it's wrong to kill without reason. I consider all people equal (until their actions suggest otherwise), I don't think you should kill me for my wallet, so I don't think it fair to kill you for yours.
Equality, fairness and justice; are what I use to reason what action I take in life, I would like to expect the same back as an act of respect by others, not the promise of an afterlife of virgins and milk & honey etc.
I would love for you to explain to me the scientific equation/hypothesis for "the right thing to do" or perhaps equality, fairness and justice. Where do you think those concepts came from? If Hitler or the terrorists of the WTC attack simply died w/o having to pay for what they did, would you not consider that unfair?