I talked a fair amount about why Kasia made the specific decisions she did on days Seven and Eight, so I'll share some more general thoughts below.
Kasia is driven by two main motivating factors: wanting to protect herself and the people she cares about, and her strong distrust of authority figures, especially those she sees as bullies.
Kasia cares about her family, she cares about her friends, and thanks to Duncan, she cares about the Grey Wardens. Duncan was the first person above her station to treat her like she deserved respect, and for that he basically walks on water. Her feelings about Duncan then transferred to the Grey Wardens in general, and so she takes her duty to defeat the Blight quite seriously, even if she feels like she's out of her depth a lot of the time. She wants to be a good person, to live up to the example Duncan tried to set for her, but it doesn't always come naturally, especially not at first, and so she sometimes does things that others would find questionable (stealing, taking borderline unethical jobs for the coin, asking people to pay her for work when they can't really afford it, etc.). But when it comes down to major decisions, things that will affect a lot of people, she does her best to think beyond her own desires.
Because of her childhood circumstances -- growing up poor and pushed around by those with status -- Kasia feels great sympathy for anyone she sees as downtrodden, and immediate dislike for people who take advantage of others. Beraht fell into this latter category, as did Zathrian (if she hadn't been able to talk him into lifting the curse, she would probably have sided with the werewolves), Branka, Jarvia, and, fairly or not, Loghain -- she had already formed this impression of him after Ostagar, but discovering that he was selling the less fortunate into slavery pretty much sealed his fate. She got this sense about Bhelen, too, but he was at least paying lip service to the idea that the casteless needed more rights; between that, her need to support Rica, and the likelihood that Harrowmont would be no better, there was never any question of whether she would help put Bhelen on the throne.
For the most part, the good opinions of her companions didn't affect Kasia's decisions -- she went directly against Alistair in Redcliffe when she choose to sacrifice Isolde, for example, and Zevran and Morrigan were both unhappy with her for destroying the Anvil, but she wouldn't push anyone so far as to leave, either -- she refused to destroy the Sacred Ashes largely because she knew most of her friends wouldn't like it.
This entry is also posted at http://owlmoose.dreamwidth.org/620429.html. There are currently comments on DW.