In my belief there is such a thing as sin and I even use the word sometimes, but only because it's easier than offering the full definition. I'm not always the best at explaining my point of view, but I think I can give a good idea of my line of thinking here.
A sin is not something that you shouldn't do because it's written in a book, but because it's just a really bad idea. I think murder is a good example of this. You see God is infinitely intelligent and infallible, unlike us he makes no mistakes. God created every person therefore if a person exists it is because they should exist, it is because God created them and he had a damn good reason for doing it. If you were to kill someone it is akin to telling God that you disagree with his decision to have created this person and I can assure you that if you disagree with God you are wrong and he is right. If a person exists it is proof that they should exist and are supposed to exist.
I will equate this to a machine. A computer is comprised of many different parts that work together to create quite a useful tool. If you were to remove a transistor from the CPU or a magnet from the hard drive the computer is not going to work as well as it should if at all. The human race is similar, although quite a bit more advanced. Like a computer the human race is comprised of many different parts, people, that work together to create a fully functional, highly advanced piece of machinery created by the almighty himself. If you start removing parts from this grand creation it's not going to work as well as it should. Being as how you are a part of this machine it behooves you to ensure that this creation works as well as possible. When you damage the machine by killing parts you are actually damaging yourself. When you hurt others you hurt yourself.
It's important to know why we should or should not do something. The reason why we do not kill each other is because we are only killing ourselves by doing so. Murder and suicide are virtually one and the same.