The Immediate Full Effect of Sin on Humanity

Cain rose up against Abel, his brother, and killed him. This is significant for a number of reasons. The one that's always struck me as most important is the almost instantaneous full effect of sin upon humanity. We have bloodshed. We have one brother killing another brother. So it's not as if gradually over the course of centuries and then into millennia, humanity sunk farther and farther into sin. And we see immediately following this Cain's unwillingness to do that.