According to the legend, Cain was the firstborn son of Adam and Eve, and therefore should have been the favored son according to tribal traditions. But Cain was a farmer and grew vegetables, while Abel, the younger son, was a shepherd. For some weird reason, "God" wanted a sacrifice to be made in his honor, and "God" preferred a blood sacrifice, and one cannot get blood from an eggplant, cumquat, or a loaf of bread, and that pissed off Cain, who had chosen a less bloody sacrifice, so he slew Abel, who probably mocked him and his sacrificial veggies.