Baptism is the defining moment because it is the defining moment. It's what sets him on his path.
And that still bothers me, in terms of the writer's choice of that event being the defining one. It's trite to make everything pivot on the choice of a baptism IMO.
Comstock the character (or the possibility of him) to me was made during the trauma of what Booker experienced at Wounded Knee, and I just think there was more to examine there as the root than whether Booker decides to take a baptism or not.
As I said earlier, I prefer the idea that in the Comstock reality Booker murdered a child at Wounded Knee (this is heavily implied) but the game Booker somehow made a choice not to kill a child, but was still scarred so much by the experience.
This type of root choice makes much more sense to me, and everything else would have flowed on and still worked, but we would have had so many more things resonating; Comstock goes sterile - Karma
Booker has Anna, but the choice is then echoed by choosing to sell her then recanting.
Echoes harvesting Little Sisters in Bioshock.
I just hate the fact that the choice 'root' is down to a baptism. Like really? Just doesn't wash with me (no pun intended).
But it is what it is I guess...