From a new book : Silver, Moshe The Weight of Gold:

We may have all read this pasuk tens of times and never paid attention to a small word : KETZ

4:3 ויהי מקץ ימים ויבא קין מפרי האדמה מנחה ליהוה.

The simplest reading of this story is that Cain brought his offering – spontaneous though it was – as an afterthought (4:3): “And it was at the end of the time period” (likely meaning the end of the season; the growing was done and the fresh produce had been eaten, and Cain brought only what was left over) “… and Cain brought an offering to God.” And in the very next verse, “And Abel also brought.” Although God does indeed favor Abel’s offering, it seems that the issue is not the superior offering of the firstlings of the flock over the musty old leftover fruits. Rather, Cain eats his way through an entire year’s produce and then decides it would be good to thank God. Abel, on the other hand, acts immediately. Before he even shears or eats from his flocks, the first thing he does is make an offering to God. What God seems to prefer is not the quality of the offering – though Abel’s gift is superior – but the quality of the one bringing the offering. The latecomer, Abel, wins out over the enthusiastic Cain, who thought up the idea in the first place. This is the traditional interpretation: Cain was selfish and gave God the leftovers; Abel was utterly selfless and gave of the very best. Cain grew vegetables and ate them; Abel cared for the sheep, even though God had not permitted humans to eat meat. Thus, Cain’s activity was selfish, and Abel’s was selfless.

Silver, Moshe (2020-11-09T22:58:59). The Weight of Gold: Attaining Your Potential Through the Lens of the Bible . Mazo Publishers. Kindle Edition.