Moses 5

 As begins to consider himself as Master Mahan, master of the great secret, we learn his motive in killing Abel - 

"...surely the flocks of my brother falleth into my hands."  (Moses 5:33)

Obviously, it was for greed and gain.

But later, Lamech considered himself to be Master Mahan, but this time he killed his own grandson - 

"...having known their secret, began to reveal it unto the sons of Adam;

50 Wherefore Lamech, being angry, slew him, not like unto Cain, his brother Abel, for the sake of getting gain, but he slew him for the oath’s sake."  (Moses 5:49–50)

Obviously because his grandson revealed the great secret which was a wicked covenant for gain.

Alma's words are true - 

"...thus we see the end of him who perverteth the ways of the Lord; and thus we see that the devil will not support his children at the last day, but doth speedily drag them down to hell."  (Alma 30:60)