Kayin had just went through a most traumatic experience, namely that Hashem openly regarded his brother over him. Anyone would be burning with jealousy and hopelessness. Yet Hashem asks him: Why are you all broken? You can still be even greater than Hevel. Hashem asks him: Why are you over-reacting? The problem of Kin'ah we all have is truly minimal. If you think someone is a bigger Tzaddik than you, then by merely doing Teshuva you automatically become bigger than him, since a Baal Teshuva is greater than even the greatest Tzaddikim. Even if your Teshuva doesn't put you exactly on the other person's Madrega, you may still be considered much bigger, since you have the difficulty and embarrassment of being a Baal Teshuva.
The true pain of jealousy most of us have is usually the Kavod the other person is getting, that people regard him as a greater Tzaddik then you. And in truth, the fact is, getting more Kavod makes him much smaller than you, since he has support for what he's doing, while you don't.
This is besides the fact that the Kavod a person gets down here can eat up his Olam Haba! Kin'ah is only good when it causes you to improve. Otherwise, it is one of the lies of the Yetzer Hara, who wants people to get depressed for no reason. Envy is ignorance!