Students of the Bible have long wondered at Cain's motive for killing Abel. Simple jealousy? Modern revelation, in the Book of Moses (Pearl of Great Price) adds back the lost information that the crime was not so much about hurt feelings and sibling rivalry as it was about something far more evil.
Cain had made a pact with Satan, being convinced that by killing Abel, he could become wealthy -- get gain.
It is terribly ironic that the very meaning of Cain's name is "possession" or "gain." That last detail I just learned yesterday, reading in the Tanakh (the Hebrew Bible). I'm not sure that Joseph Smith the Prophet had any idea of that connection, when he brought forth the lost details of the Book of Moses to the world.