Why did Cain kill Abel?

As the older brother, Cain was to follow G-d, anything else was not an option. But, the younger brother, Abel, could make a choice to follow G-d or not. (Ex. 34:20) So, when they came to bring an offering to the L-rd, (called Mincah in Hebrew, translated as 'grain offering') this was Cains' opportunity to step up to the plate, since he was a tiller of the fields. But, since he had no respect for G-d and just offered of his fields, NOT the firstfruits as his brother Abel did - his offer was rejected. He therefore took out his anger on the one who choose to follow G-d out of a willing heart...his own brother. Shalom!