Why did God protect Cain after killing his brother?

I was reading the bible and on the book of Genesis it says that Cain killed his brother out of Jealousy. But then after Cain kills his brother Abel, God tells Cain he will no longer be able to grow crops. And he put a mark on Cain so that anyone who killed him would be punished up to seven times. Why didn't God curse him to just have bad luck instead? And who exactly was Cain afraid of being killed by if at the time the only people alive was his parents and himself? And does this mean that Cain's wife was his own sister who was born to Adam and Eve later on? Somebody please explain to me because the bible doesn't go into much detail about this?