It wouldn’t have been all that hard for Cain either. He was a fruit and vegetable farmer, and Abel specialized in livestock. They could have cooperated furnishing Abel with a larger variety of food to personally consume and furnishing Cain a sacrifice of the kind that pleased God (it was a foreshadowing of the Lamb of God which is why fruits and vegetables would not do). It was Cain’s stupid ego and pride that was his downfall.
Pride - the root of all evil... starting with Satan.
He was probably also the very first vegan and let everybody know it at every opportunity.