When thou tillest the ground, it shall not henceforth yield unto thee her strength; a fugitive and a vagabond shalt thou be in the earth.God cursed the fruits of Cain's labor because Cain killed Abel, his brother. No matter how hard Cain worked the ground it still wasn't going to yield up to it's potential. This was also the case for Adam and Eve, Cain's parents. They fell from God providing everything, to having to work for everything. Sin is sin and it's all relevant.