The part I'm talking about is when Eleanor gives you control of a Little Sister, so you get to see the world through the Little Sister's eyes. At first we figured that she was in some kind of training facility for Little Sisters. Everything is all soothing, soft colors, clean, a few toys here and there, and some scatterings of rose petals.
Then we come upon an "angel", who has "wings" of glowing lines, and more rose petals. The angels looks happy and peaceful as s/he sleeps. I said to Toly, "The rose petals look like bloodstains on the floor."
Then, when the Little Sister starts harvesting ADAM from the "angel", we see what's really going on: the rose petals are bloodstains, the angel is a mangled corpse, and everything is as dark and dysfunctional and horrifying as, well, Rapture.
So the truth is finally known: the Little Sisters are brainwashed or conditioned to see the world in the peaceful, soft look, because otherwise they'd go insane as they harvest ADAM from murder victims, in a city falling down around them and filled with psychotic killers. I noted that technically the Little Sisters probably are "insane", in that they experience an extreme delusion about the world, because of what Gill Alexander (? Or was it Sinclair? I'm not sure any more) did to the girls when they were turned into Little Sisters.
For my money, BioShock and BioShock 2 have some of the scariest villains and ideas in games today. Oh, there's plenty of killing and the like, but Andrew Ryan's libertarian utopia-turned-nightmare and Sofia Lamb's socialism-on-steroids are freaking creepy. Lamb's idea to turn Gill Alexander into the ultimate utopian, by completely supressing his sense of self and humanity so he can only "serve the common good", gave me chills.
On a side note, Toly tried BioShock 2 multiplayer, and found it underwhelming.