What you do changes your journey. Harvest Adam from the Little Sister and you'll access and power up your plasmids. Their sacrifice gives you the hit you need, but you're participating in the exploitation. What separates you from Ryan? Is it a justifiable means to an end? Do you care? Perhaps you do. So instead of harvesting her, you choose to save her - exorcising the mutation. But where's the pay-off? You've still got a whole host of Splicers roaming the world ready to kick your ass and you're not getting access to the sort of powers that make Rapture, an already pretty demanding place, any easier. You'll have to think harder, and play better. And, Irrational assure us, you will also be rewarded in a different (and as yet under-wraps) way, by Tenenbaum, the geneticist whose research began the enslavement in the first place.
Well, you'll have to get past the Big Daddies first. Their remit is simple; protect the girl at all costs. So as long as you leave them alone, you'll only have to evade the attention of, well, everything else in Rapture, like crazed lunatics whacked out on Adam - "You're not one of us!" one nurse screamed, as she took a run at us.
