Cain and Abel

Adam raised a Cain. That's a line from a great Bruce Springsteen song. It makes the point that even Adam, the first man created by God, was unable to raise perfect children.
But why was Cain so bad? I mean, here we are not more than a page or two into the Good Book and we already have our first killing. What happened? What went wrong?
Well, it seems that the problem was jealousy over God's favor. Cain was a farmer while Abel was a rancher. And when they made their offerings to God, God decided for whatever reason to play favorites by rejecting Cain's offering of fruits and vegetables while smiling upon Abel's offering of a fatted calf.
So what are we supposed to make of that? Is God a bloodthirsty carnivore? Why did he play favorites? Was it another test? One that we would fail yet again?
Immediately after rejecting Cain's offering, God spoke to him giving him somewhat of a pep talk and warning him against sin. But what was so sinful about what Cain did? Why was his offering rejected?
But regardless of what God said, it only made Cain more angry because he next took Abel into a field and whacked him. Then, later on, when the apparently non-omnicient God finally finds out, Cain delivers one of the first classic lines from the Bible: "Am I My Brother's Keeper?"
So God lays down another harsh punishment causing him to be doubly exiled.
At this point we get to meet Cain's wife, and one might wonder where did she come from? And what about all those people that Cain is afraid will try to kill him once he is forced to wander the land?
So was Adam not really the first man if there are already all of these people out there populating the earth? Eve's third-born Seth hasn't even been announced at this point, Abel is dead and Cain is afraid of what the Others might do to him. So apparently we are not getting the full and complete story here.
But, as it turns out, we won't have to worry about these other people for long because very shortly they will all be drowned.