The answer is: Murder

Gen 4:8

When we hear the name Cain, our first thought usually plunges into a tale of betrayal and loss, specifically through the act of murder—the dreadful sin that stained his legacy. The story of Cain striking down Abel unfolds not just in the snare of sibling rivalry but reflects broader human themes. Why did Cain commit such a heinous act? Jealousy half-driven by feelings of inadequacy in God's eyes, and a deep-seated rage that morphed into fatal violence.

Interesting tidbits about Cain is that he is often viewed symbolically in different discussions. He represents the aftermath of unchecked anger, which can lead one to abandon the allure of righteous behavior. Additionally, did you know that in various cultures, Cain's name means "acquired" or "possession"? This aligns intriguingly with how he sought approval from God through his offerings, desperately wanting to “possess” his maker’s favor—this only served to amplify his discontent when Abel’s offering was embraced and his own was not.

Another critical Biblical echo to consider is that after Cain's act of violence, he was marked but not obliterated, a testament to divine grace even amid their failures. This protective mark has sown seeds of various interpretations through generations, crafting debates around mercy and accountability. Cain’s narrative drives home powerful lessons about human emotion and moral choice in all its complexity—from despair to deep remorse, compelling us to consider choices beyond violence in our own lives.