Sure enough, as God warned Cain, his jealousy and selfishness eventually led to further sin. Cain murders his brother.
I think in order to stay consistent with its objections in verse 3:9 and elsewhere, the SAB really ought to mark every passage in which God asks a question. If God knows everything, why is he asking questions? Of course, if anyone did indeed have a problem with this, it should be especially clear in this passage that God is not asking questions that He doesn't know the answer to, as He proceeds to essentially immediately answer His own question in verse 10. God's looking for a confession from Cain, but all He gets is a smartass retort, "Am I my brother's keeper?" (It's an interesting parallel in the KJV that Abel was a "keeper" of sheep, and Cain's final word on his rivalry is to point out that he's not a "keeper". However, I looked, and this parallelism is not in the original Hebrew, so it's just an oddity of this particular translation.)
Anyway, for committing the second murder, Cain gets a curse. It's not clear at all to me what portion of this curse is punitive and what is just the natural consequence of his transgression. Being "a fugitive and a vagabond" seems like the sort of thing that just plain happens when you're a murderer, since you will eventually have to flee the scene of the crime, and it seems somewhat reasonable to assume that all of the people in the world at this point, however many they are (could be several, although probably not many) all live in the same general area, and are near relatives of his brother and him. So, he has to run away. As for the ground somehow being even more cursed for him than it was for his father in the previous chapter, I don't know what that's about. Perhaps since they are likely living near the garden of Eden, the ground in that area is nicer than in outlying regions. That's pretty speculative, though.
So Cain proceeds to complain about his punishment, and claims that people will try to kill him. I don't really understand why he is so certain this would be so, nor why God seems to agree that there is a risk and protects him. Actually, the whole "mark" thing is rather odd, and I wonder what sort of mark a person might have that would say to any person he met, "Hey, don't kill this guy!" but apparently, such a mark existed. (I don't buy that this has anything to do with dark-skinned people, especially since it's likely no descendants of Cain were on the ark.) Most likely, God is interested in preventing vigilante justice; Cain has already received a punishment from God, nothing more is needed.
Cain also says to God, "From thy face shall I be hid." This is not to say that God shall be unaware of the doings of Cain from this point onward, but rather that Cain is turning away from God, so to speak. God has an eye on everyone in the world, but not everyone has an eye on God. Sometimes my children, when they get angry, will turn their backs to me, effectively hiding themselves from [seeing] my face, but not becoming invisible. This is the same sense that is meant by "out from the presence of the LORD" in verse 16.
Starting in verse 17, we get a sort of life story and genealogy of Cain. Yes, Cain had a wife. No, the Bible doesn't say where she came from. Most likely, she was a sister, but as some others have pointed out, she could have been a niece (although such a niece probably would have been the offspring of two of Cain's siblings, so it hardly makes her less of a close relative genetically). They settle down somewhere far away from the rest of civilization and start a family.
They also start a city, which the SAB is right to be confused about in light of Cain's curse to be a "fugitive and a vagabond". Such people don't make cities. Some possibilities come to mind, though. Cain did remain a fugitive from the other part of society for all of his life, I think we can pretty safely assume. The curse may have simply been that he had to leave all that he had previously held dear and relatively become an outcast. Another couple of possibilities both have to do with that part of his curse being temporary. God said he'd have a hard time with planting "henceforth", but doesn't really give a timeframe for being a vagabond. Either eventually it just ran out and was no longer effective (not all natural results of sin last forever), or if it was a punitive sentence set on him by the Lord, then he simply refused to comply. In the latter case, building a city was yet another act of defiance, which may well fit with his character as we know it.
I guess I'll wrap up this post with a few comments on Lamech, Cain's great-great-great-grandson, of whom for some unknown reason quite a bit is said. Perhaps the reason is that he seems to be exceptionally evil, although he does seem to have okay kids. Lamech is the first noted polygamist in the Bible. I already mentioned my view on the Biblical subject of polygamy at the end of this post, and I think all that needs to be added is that it's hard to see (at least for me) that the Bible is setting this guy up as an example of righteousness and right living. Lamech makes a claim at the end of his story that although he is a murderer, in the manner that Cain was promised to be avenged "sevenfold", he would be avenged "seventy and sevenfold". The SAB calls this absurd and unfair, and I totally agree, but I also point out that this is Lamech talking about himself, not God saying anything about anyone at all.
Aw, heck, I'm going to consider that the end of the chapter for me. There's really nothing to say about the last two verses that hasn't already been said about the first two in yesterday's post. Have a great weekend, everyone!