While Atlas doesn't end up being as he initially appears, he is the first non-hostile person you interact with in Bioshock. His pleas for you to help him save his family pushes you through early on in the game and gives you a sense of purpose beyond fighting the crazed splicers. You push through all of the splicers, get to the submarine bay, and see Atlas saving his family. It's all going to work out!
Aaaand then Andrew Ryan blows the submarine up. Suddenly, you get brought back down to the hopeless reality of Rapture. Although you learn that the whole situation with Atlas's family was to manipulate you, this is a completely unexpected moment on the first playthrough that leaves you with a bit of a pit in your stomach. Atlas's anguished yells only add to it.
Once this plan fails, you lose the hope that came with your initial objective and the rest of the adventure is a blood-covered, vengeance-motivated journey where killing Andrew Ryan is the one and only goal left.