That's exactly the same issue I had. I was just a guy (possibly also an asshole depending on playstyle), heading off to kill another asshole (so we were told), by yet another asshole. Stealing the little sisters from the big daddies felt wrong almost all the time. They're just doing their thing, and plasmids weren't really effective enough to merit needing all that ADAM. The guns worked just fine on their own.
I lost all motivation.
Bioshock 2 on the other hand, gave you a purpose. I loved it. You were there to save someone dear to you, with revenge as a side dish. You could actually be caring to the little sisters, because you could protect them to get what you wanted as an added benefit, and then send them off on their way. Plasmid combos were a lot more effective, and I used them nearly all of the time with their complimentary weapons.
In Bioshock 2 you were a savior (yes, even if you were evil) and a mentor, rescuing someone who dearly needed you from a false savior who would twist her for their own selfish ends.
In Bioshock, you were just another prick in a sea of them.