Read 3-5. Cain’s offering was rejected whilst Abel’s was accepted. Why? Well, the text hints at it (“some of the fruits of the soil…fat portions from the firstborn”; Abel was offering his best but Cain was simply offering what was near to hand) but we’re told explicitly in 4 that Abel made his offering with faith. Presumably Cain made his offering from unbelief, being motivated not by love for God but perhaps by love for self. It’s hard to know exactly what was motivating Cain but the traditional interpretation is envy, fuelled by pride: I think Cain wanted to be the better worshipper; he wanted to be commended more than Abel. So, when his offering was rejected, when he didn't get what he desired, or thought he needed—Cain then made a moral evaluation. Cain made a value judgment of the situation and said: “this is wrong. I am displeased by this.” Of course, the unstated idea behind Cain’s anger must be something like this: “God has got it wrong; God is being random, capricious, cruel, and unfair. I’ve tried to worship God but look at how He has responded.” So Cain becomes very angry, because anger is caused by a moral evaluation in which you say of something: that’s wrong.