Booker's potential to be a monster is very high. He's already there before the events of Infinite. Elizabeth alone gives him a reason to re-think his actions. When Elizabeth and Booker find themselves confronted with a man who may well give them away he's given a choice: Kill the man, or spare him.
If he kills him he furthers Elizabeth's reasons to think him inhuman. If he let's him go, they may get discovered and quite possibly end up killed themselves.
Booker looks at Elizabeth. At her face. He only sees one choice.
Bioshock will force you to make decisions based on whether you want to make the game easier for you, or whether you want Elizabeth to think you as something besides a heartless killing machine. What will you choose?