*SPOILERS*
On the surface Isabella seems like an Immoral selfish person and her actions during the coarse of Dragon Age 2 undoubtabley are.
But if you go deep inside her (no pun indended) you will find that actually there's more to her than it seems.She just had an extremely traumatic past that taught her a "survival of the fittest just be selfish" personallity.She's actually a sweetheart at heart.
She was sold and used, treated like an object, so after she got out she valued her freedom and was emotionally confused as to where she stands ethically wise.After these events she thought everyone was out for themselves and will hurt her feelings if she lets them close, so she didn't let anyone into her life.
As much of a ****** she seems, she couldn't sell all those slaves before the events of DA2 because deep down she's a good person.
She also possibly made a friendship (+50 friendship) or I know you're right so I hate you relationship (+50 rivaly) with Hawke that made her come back with the tome since she felt that she owes it to him/her.She isn't really morally deprived as she makes everyone think(including herself), not really.
She becomes aware of her mistakes and the wrongs she did if in her final companion mission if you kill Castion.She realises that her actions have concequences and actually turns into a somewhat morally normal person.She finally lets her friends and potential love(Hawke) into her life and makes connections and loyal friendships with people.
Her character development is amazing as she changes as a person over the three acts.
She is my favourite Dragon Age character and I bow to her writer in respect for making such an amazingly complex character.