Play nice and you save the Little Sisters. Hooray! But then you also - apparently - whisk them away to small town suburbia, put them through college, finance their weddings, play grandpa to their kids and die in their arms. Of course.
Play mean and you hurt the Little Sisters. Boo! Hiss! But then you also - apparently - decide to marshal an army of flying mutants to take over the world using a nuclear warhead you just happened to find floating around in the ocean. Come on, you must have seen that coming...
BioShock is not black and white, but a masterpiece of grey. Nothing is clear; nothing is simple; nothing is defined. Above all, your character and your morality are whatever you make them, slates left purposely blank and ready for interpretation.
Which is why these extreme endings feel like such betrayals. Just because we harvested a Little Sister or two - hey, Rapture's a dangerous place! - does not mean we deserve to be cast as crazy, megalomaniac Bond villains. And just because we rescued every Little Sister - for the Achievement, while killing all the poor Big Daddies in the process - does not mean we deserve to be cast in a tearful Lifetime Movie of the Week, either.