Cain should have cared Abel was his twin brother, he didn't really know how to control himself, jealously seems to be the reason he got made at Abel. You should care about your fellow of mankind because it all comes back to you in some ways. Cain had the mark not for what he did, it was to protect him from some people who would want to kill him, people seemed to have not understood there were others beside Adam's family, I know pun was intend I couldn't resist this one I sure God will forgive me this one. Anyway there he was going had others who were also in the world, these are the ones sent out to replenish the world, yes, there was others, Adam was in the garden when he got a mate created by God so he would be without a companion, just as others. Male and Female created he time, and sent the out into the world to subdue it and replenish the world. There are mentioning about this in different places in the Bible.

Water was in the deep and God divided the waters but there seems to be more to understand. You know he created the ones who were really called Watchers before man or angels if you would rather call them this. They were a different flesh, a different kind in other words. We are like unto them but not the same nor ever are supposed to be, that how the mess came about because of the watchers wanting something they didn't need, and God told them watch and not to touch.