What made Cain so angry?
And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering:
But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell. ...
And Cain talked with Abel his brother: and it came to pass, when they were in the field, that Cain rose up against Abel his brother, and slew him. Genesis 4:4-8