Of course, Cain's original crime was one of the worst things a person can do. Not only did he kill someone, he killed someone that he was supposed to love. This is a terrible thing.
But then he went and made it worse. What Cain should have done was to immediately confess what he had done and beg God for forgiveness. But instead what did he do? Instead of confessing, he tried to lie to God. By doing this, he did not show that he was truly sorry for what he had done. Instead, he showed that he still cared only about himself, trying to avoid being punished for his action.
You can also say he continues to make it worse when he complains to God about his punishment. Once again, he is not showing true remorse.