"32 And Cain went into the field, and Cain talked with Abel, his brother. And it came to pass that while they were in the field, Cain rose up against Abel, his brother, and slew him.
33 And Cain gloried in that which he had done, saying: I am free; surely the flocks of my brother falleth into my hands.
34 And the Lord said unto Cain: Where is Abel, thy brother? And he said: I know not. Am I my brother’s keeper?
35 And the Lord said: What hast thou done? The voice of thy brother’s blood cries unto me from the ground.
36 And now thou shalt be cursed from the earth which hath opened her mouth to receive thy brother’s blood from thy hand.
37 When thou tillest the ground it shall not henceforth yield unto thee her strength. A fugitive and a vagabond shalt thou be in the earth.
38 And Cain said unto the Lord: Satan tempted me because of my brother’s flocks. And I was wroth also; for his offering thou didst accept and not mine; my punishment is greater than I can bear." (Moses 5:32-38).
This was not a crime of passion, it was calculated murder. Cain's purpose in seeking out his brother Abel was to kill him, to deprive Able of his life. In his post-murder phase Cain gloried in his power. He had power over life - like God. He could take Abel's flocks and be rich. Then the Lord came to Cain asking him where his brother was. Cain's reply? "I don't know. Am I supposed to baby sit him? Am I his shepherd? Am I his keeper? I am a keeper of secrets, not my brother." Then when the Lord exposes Cain for what he is, a murderer, Cain starts blaming Satan, the temptation of riches, and anger. "Satan made me do it. I was blinded by the rich, white coats of my brother's sheep. I wasn't myself, I was in a fit of rage." These are all common excuses people still give when they get in trouble. It's always someone (or something) else's fault. Cain did murder for money but for a number of years before this he had loved Satan more than God. He did not just see Abel's flocks one day and decide to kill him, Cain was wicked already, had covenants with Satan, and already had a plan to kill Abel.
What I want to focus on though is Cain's question: "Am I my brother's keeper?" Cain was not simply answering in annoyance - "I don't know where my brother is!" - he was revealing his callousness towards his family and other people. Cain revealed his self-centeredness and his selfishness. He was Satan's protege and as the protege he adopted some of Satan's characteristics, most predominantly, pride. Cain thought he could hide his sin from the Lord - that was a manifestation of his pride. He thought that he was not responsible for his brother, especially because Abel's sacrifices were accepted but Cain's were not; Cain felt offended by the Lord and by Abel. Cain took every opportunity of aggrandizement. He sought power and glory and riches (sound familiar? Satan sought the same things in heaven) at the expense of all else. No one would stand in Cain's way, especially not his brother, who was the favored son.
In some ways this sounds much like the story of Joseph who was sold into Egypt by his brothers. His brothers were jealous of Joseph's favored status (which was based in part on his righteousness); they wanted to kill Joseph but were convinced not to by Reuben, the oldest son. Maybe the brothers hoped to gain Jacob's favored status; maybe they just wanted to kill Joseph because they were evil people and not necessarily just to get gain. In any case, there are repetitions of themes throughout the scriptures - wickedness and righteousness are found in cycles in the Book of Mormon and Bible. Cain was only the first of many murderers. But because he was the first he became Perdition and (future) lord over Satan.