8 Now Cain said to his brother Abel, “Let’s go out to the field.” While they were in the field, Cain attacked his brother Abel and killed him. Genesis 4:8
The Lord had clearly warned Cain about what could happen next. He knew the anger and jealousy that Cain had built up inside him. I don’t think we quite understand the extent to which he was angry and resentful. They say the coverup is worse than the sin, and it’s true in this case. To mask over his hatred for his brother, Cain killed him. I suspect there were more incidents like this throughout their lives, and the insufficient sacrifice was the straw that broke the camel’s back, so to speak. And it may be that the Lord had been warning him over the course of those years. We are just privy to one important incident.
The point is, anger and resentment can build up over time if you’re not careful. Keeping those small incidents in check is important. Letting the incidents pile up and escalate in your mind is unhealthy.