way to go on the spoilers in the title, yet masking them on the message haha
but anyway, i think Booker was so over taken by his guilt that when he accepted the baptism, it not only freed him from it but it also went to his head. he was set free from his sins and the feeling was such a weight lifted off from him that he took it far more literal than anything else. Booker was done with his former self and wanted to become someone else entirely and that spiritual freedom gave him the egotistical boost to do just that.
its not oncommon in real life for things like that to happen. spiritual conviction has turned alot of seemingly normal people into fruit cakes. look at Kirk Cameron for example. while he is no evil tyrant comparable to Comstock, but he can be extremely annoying these days. he started out as the lovable Michael Seaver from Growing Pains, but found spirituality (wasnt taught properly) and became this really annoying, judgmental thorn in everyone's side and made it difficult to work with on set. he threatened to walk out on the show unless the writers made his character different, and all sorts of craziness.
thats for sure! but i really like that about this game. its not so much a "its up the gamer" cop out answer, but it begs the player to pick each other's brains as to what they think about it!
You have to understand back then (1870 - 1912), it was widely accepted that negroes are lesser beings. You can't really ask "Why does he become a racist", if what he is doing is considered standard.