Qayin kills Hevel because Hevel's offering is fatter than Qayin's, and God favors it.
Qayin wanted Hevel's place with God, not Hevel's greater material wealth. It wasn't what Hevel had that compelled Qayin, it was his closeness to God that he envied. Hevel is killed because he fulfilled the obligation to appreciate God's bounty more exquisitely than did Qayin.
If there is a lesson here, it is that it is better for the abundance to be delivered to God by the collective before it turns into an arms race.