The Keli Yakar explains the incident of Cain and Abel, from a different angle. He writes that they had an ongoing debate regarding man’s ultimate purpose.
Cain was of the opinion that man should live for today, as he did not believe in an afterlife. He was a farmer and connected to the ground. He saw this world as permanent, and nothing followed it.
Abel, on the other hand, concluded that this world was temporary, and a greater world awaited us. We needed to apply ourselves to deserve to have a part of that world.
He dealt with shepherding, as the sheep were movable items, and temporary, just like this world. Cain held one thing over Abel. He was the one who initiated making an offering to Hashem before Abel. It annoyed him that he stole his idea, and outdid him by offering a higher quality offering than him.
This was his motivation in killing him. They had very different philosophies on life, and Cain allowed jealousy to consume him, to the point that he murdered his brother.