The Bible scholar Ronald Hendel analyzes the tale of the first murder:
In the biblical story, Cain has several motives for murdering his brother Abel. The most obvious motive is jealousy that God accepted Abel’s offering over Cain’s. But this is more than simple envy: as the firstborn son, Cain holds legitimate authority over his younger brother. . . .
Cain . . . is the family’s firstborn son, offering his sacrifice first. And by the normal rule of family authority, his sacrifice should be accepted first. But it isn’t—God defies expectations and accepts only the younger son’s offering. . . . The normal hierarchy of the firstborn and younger child is turned upside down.
Read more on Bible Odyssey: http://www.bibleodyssey.org/passages/main-articles/first-murder.aspx