I'll answer your question with another question to put things in perspective. Pretend you don't believe in God, just for a second. Not a militant anti-religious nut, just some regular schmo who happens not to be religious.
Got it? Good.
Now, pretend you've got a family. A wife, and two kids.
Got all that?
Okay, now say burglars break into your house, tie all of you up, beat you half to death, rape and kill your wife while you watch, and finally walk out of the house with both your kids and $5000 in stolen property.
So far, so good?
Now, as you lay there bleeding to death, being the hypothetical aetheist that you are, would you simply say, "Eh, what can ya do. This was all the result of an accident anyway."?
In other words, people define what is right and wrong. People have emotions, and when those emotions get hurt, after a while, we found out what actions hurt us and decided we shouldn't let people do those things.