What the fuck happened that turned booker into fucking charles manson aka comstock, or at least see the timeline in the universe where he decided logically each step to make a floating city and then kill off everyone on Earth. Be cool to play as him and just make those tiny little "well, that seems like a good idea" steps that leads to columbia being built, taking off, and starting that whole philosophy.
I was also bothered a bit by the game suggestion that somehow stopping booker from being baptized/born again stops the creation of comstocks across all universes. If there are an infinite number of universes, it doesn't matter what you do - an infinite number of comstocks will still do that thing and in fact an infinite number of other people will do something similar: comstocks wife will make her columbia and nuke the earth, anna will do it, hell there are an infinite number of universes where the twins destroy humanity.
The second you decide an infinite multiverse is real and/or you can cross between them, it literally doesn't matter what you do because every possible configuration is real and you changing them does nothing because you changing them is just an infinite subset of configurations, along with you not changing them. Which is why the infinite multiverse theory sucks and I hope is not right.
Well, since we are ok with no spoilers, I'll answer things as best I can, since a lot of this is still fresh in my mind. If i have to edit it into spoiler tags later I can.
It takes a little reading between the lines, and listening to a lot of the phonographs, but what i gathered about Comstock was this. When Booker went to the baptism after the Massace at Wounded Knee, he was full of doubt and self-loathing over what he had done. The Booker DeWitts are the ones who rejected the baptism, they internalize their pain. This leads him to the pinkertons (more bloody violence) which leads a spiral of alcohol and gambling.
The Comstocks are the ones who take the baptism, they come out of it truly believe that their sins are forgiven (the blood on their hands is washed away). they externalize their pain: "Its not my fault, its the world that is rotten, the Lord has saved me!" This leads them to instead pursue a nationalistic/religious life, where they become wealthy/powerfully connected within the growing American Govt. They meet Rosalind Lutece, who is working on her science, leading to the idea of the floating city, the jewel of the great America. But when Columbia is involved with the (IIRC) Boxer rebellion incident, the American govt tells Comstock to come back and he tells them to fuck off. Its at this point that Rosalind has, with his funding, gotten her technology to where they can open the tears and see other realities. Comstock uses this and starts his decent towards 'the world shall burn' schtick, since he now feels not even America can live up to his impossible standards. He needs a child to continue what he has started, because excessive passage through the tears has aged him and left him sterile. Thus the plot to get Anna from the Booker DeWitt reality. Basically, Comstock is a jerkass who blames everyone else for his mistakes and decides to destroy the world because it cannot be what he wants.
As for the ending, the way I see it is that at the moment of the 'Final Baptism", 'our' Elizabeth has her full power (We were the only reality to destroy her Siphon, thus giving her full strength). This allows her to see across all the various realities and change things by her will alone; so she finds all the Booker's who would accept the baptism and become Comstock. She uses her power to, how do I want to say it, tie them to that exact moment; in essence 'our' Booker is now all of those Booker's in one being (represented by all the other Elizabeth's who appear). 'Our' Booker, devastated by all the realizations he has just been shown, allows them to drown him so that none of those Comstock's ever live. This allows her to cut off all those realities from ever existing, which also causes the Elizabeth's to disappear, since they will never come to be.
Side Note; I will say that I don't believe 'our' Elizabeth disappeared tho. They certainly were not holding back on the punches at that point, and having 'our' Elizabeth not disappear but instead have the screen fade to black is a copout. I'm too goddamned Genre Savvy to take that at face value 2K. At that moement, she is practically God, you can't tell me that she just up and poofs too!