“If you’re doing what is good, shouldn’t you hold your head high? And if you don’t do what is good, sin is crouching at the door—it wants you, but you can rule over it.” What important lesson is God teaching here?

Certainly, Kayin was not doing what is good…he allowed the evil inclination (yetzer harah) to take over and he ends up attacking Hevel in the field and kills him.