Just so I'm clear, Booker is Comstock (technically right?). How does he end up selling Anna/Elizabeth to himself? The scene where her finger is cut off.
So, basically, if Baptism occurs he becomes Comstock. If it doesn't occur, he becomes Booker?
But then future Elizabeth drowns pre-Baptism booker to make sure he doens't become Comstock? My head hurts.
Yes, Booker is Comstock, and not even technically, but actually Booker from another universe.
Comstock was rendered sterile because of the use of the Lutece Tear machine, but he wanted a heir to Columbia, and wanted it to be of his own flesh, hence he went to Lutece who suggested to utilize his brother from our Booker's universe to get him to sell him his own daughter. Booker, full of debt, and feeling he's not a good parent, accepts, however he regrets this and goes on to stop them from leaving. This is when Elizabeth loses her finger and gives her powers to create tears.
And yes to your second question. Booker, who had arrived from the
Massacre of Wounded Knee felt guilty because he himself slayed Native Americans because he was called an 'Indian supporter', he was offered a baptism, to cleanse him of his sins and be reborn (hence the change of name to Comstock). Comstock, who took the baptism, lost his guilt, yet he did not think that what he had done was wrong, he took it as a necessity to be reborn (or something similar), and so he glorified it (which explained the the small writing when you entered Columbia). While Booker, our Booker, considered himself to be irredeemable, that simply washing himself at a river wouldn't erase the atrocities he had done, and so walked away from the baptism. He was able to live with it, and actually grow from this, becoming a better person (way better than Comstock to be honest).
EDIT: Damn, forgot the last point. Since Elizabeth now has the knowledge of knowing every universe and every outcome, she understands that in an infinite amount of universes where Booker decided to take the baptism, the world will be at war with Columbia, and so decides to end his life in the universes where Booker decides to become Comstock (this is referenced by the appearance of several Elizabeths before Booker is drowned). Booker also understands this and allows himself to be drowned. It was said earlier in the thread that this is really not our Booker we're seeing being killed, but a Bookers who decided to become Comstock (I'm still trying to figure this out, mind you), which is referenced because when each Elizabeth grabs Booker and say: "It's Comstock", and the other says "It's Booker", Booker just says: "No, I'm both", which all in all could mean that this Booker could actually become a Comstock. Elizabeth to make sure that there is no possibility of a world where Columbia destroys the world, drowns this Booker, creating a time paradox, killing that universe's Elizabeth.
Anyway, what happens after the credits, is the Booker that did not decide to become Comstock, and since the universes where Comstock lives do not exist, he can keep Anna.
Pretty cool, huh?