The story of Cain and Abel is another strange story in the bible, that was probably meant for a lesson, as well. Most of the bible, in fact, is lesson telling. The story showed how Cain turned against God and how men became evil beasts.
We are supposed to treat people equally and pretty much everybody knows that. Killing a person is a terrible thing. I couldn't imagine ever doing such a thing, but sometimes the devil grabs a hold of people a takes control of them. Though a lot is up to us, too. We decide our own lives. We can by weapons and take revenge on people or we can take a deep breath and forgive a person for their mistakes. The more we make wrongs rights, the better the world will be.
God does care about the killing that goes on in the world and it hurt God when Abel was killed, but God couldn't bring Cain to his death. God cares to much about his creation. God figured it would be better to send Cain away from his family, so that Cain would have a chance to start a new.
I don't know for sure where Cain's' wife came from. I'm not totally familiar with the story. I do remember hearing about this, though. It could be that God created a woman for Cain to start with and it just wasn't mentioned. The bible is full of mysteries and human mistakes, but we can get the idea, for the most part.
There are different stories in the bible that talk about men with more than one wife, but who were the men with more that one wife. They usually represented the evil side of the story. People like Moses only had one wife. Jesus didn't have any at all! At least things are for the better in these days. Now men can only marry one woman.
In the days of Cain and Abel, there was no heaven. It wouldn't be until Jesus came much later and died to save people from sins, that people would start going to heaven. I don't think that there was even such a thing in that time. Though, I'm sure that the descendants were told about God and about Adam and Eve. After all was said and done, life went on and the people spread throughout the land.
I don't know what provokes a man to kill another. I don't understand why people need so much. I only know that God loves all life in all form and desires peace and happiness. If we could stop thinking about ourselves long enough, then we could focus on loving each other a building for a better future.