Cain: the first murderer.
This guy is Adam and Even's firstborn son, but he sure doesn't live up to the title of third man on earth. Nope, he ups and kills his brother. His motive? Jealousy—God liked Abel's offering better than his (4:4-5).
Cain's crime is a little lesson in the mechanics and consequences of violence. The first consequence: the blood of murder victims is soaked up by the soil and cries forth to God for justice (4:11-12). Yep, murder is thus the source of pollution. And about ten generations later, after humans have done more and more (and more) violent acts, God declares the earth "corrupt" and literally "filled with violence" (6:11-13). At that point, only a flood—the flood—can wipe clean the violence-polluted earth.
And Cain was the one who started it all.
God sentences Cain to life as an eternal wanderer, which means no home, no family, no livelihood, and no protection. Orestes and Oedipus had the same fate, and to be honest, it seems pretty tame to us.
But the deity gives in to Cain's plea that his punishment is too harsh (4:13-14); instead, he brands him with a mark, a sign of his divine protection (4:15). Evidently, Cain was also the first person to get a tattoo. The next best thing to a witness-protection program is a deity's oath: "Whoever kills Cain will suffer a sevenfold vengeance" (4:15 NRSV).
When all is said and done, Cain ends up doing pretty well for a murderer. He lives in a land called Nod, gets married, has a son, and builds his very own city (4:16-17). His ancestors will learn to raise livestock, invent music, and make copper and iron tools (4:20-22). What are we supposed to make of these consequences?
Okay, so God seems to botch this first murder trial. The person who murders Cain is threatened with "sevenfold vengeance" (4:15). Translation: the killer will be killed along with six of his relatives. And Cain, who's himself a killer, not only lives, but lives safely and happily. How's that fair? Could this be why violence is out of control right before the flood?
After the flood, God finally sobers up and issues a clear law to put a stop to this spiral of violence. The principle is simple: life for life. You kill and you'll be killed (9:5-6).
Abel is the second-born son of Adam and Eve, and the first of many second- or later-borns in Genesis to receive special favor over their older brothers (think Isaac, Jacob, and Joseph). This is actually pretty backwards in a culture where firstborns are supposed to have the trump card.
In Abel's case, God prefers his offering to his older brother Cain's. Why? Maybe because Abel, as the first herdsman in the story, is also the first to sacrifice an animal to the deity (whereas Cain, who's a farmer like his father Adam, offers only grain). Who doesn't prefer BBQ to Wheaties? Later, the smell of Noah's sacrificial animals roasting on the fire entices the deity into swearing off destroying humans ever again (8:21).