On the contrary, she has been brought up to believe the world and everyone is ruthless, and that is how everything should be. That's what he likes about Elend - he is not ruthless, and yet he is kind to her. She would like to despise him at first, but she - she finds out she can't. Because he cares. Because her world is upside-down. Reen said nobody could be trusted, Kelsier proved otherwise. If she can question that very 'truth' of her universe, she can question anything. Including her own way of being. Elend is so different from the people from the slums. She fascinates him. And she can't be indifferent to that.
Now, to the first you said - that's human nature of the very same characters. We, as human beings, always strife about what is 'good' and what is 'bad', and we always try to justify things saying 'oh, but that person did that because he was bad, or crazy, or psychopatal, human nature is being good', or so. It is not Brandon forcing the issue. It's the moral of the characters themselves forcing the issue. You'll see something more interesting about it later with Sazed.