Cain's punishment for disobeying God and killing his brother was he would not be able to grow anything from the ground, he should be a fugitive and vagabond, and he was marked.  The Lord said that whoever slayed Cain would have vengeance on him sevenfold. Cain felt that his punishment was greater than he could bear. Cain struggled to accept God's punishment because he knew that with the mark that was put on him anyone that would find him would kill him.