It's because of 3 main reasons that are readily apparent by then.
1: She's selfish to a fault. Her way of thinking is that if the Warden wastes his time, charity, and energy on those who can't help themselves, then the warden is too soft-hearted to do what needs to be done or some such.
2: She is narcissistic. She loves herself to death, and thinks that people without the backbone to do things for themselves are worthless. Her way is the only way she can see people doing anything, and deviations from that are signs of weakness.
3: She's mean-spirited. She gets her kicks out of people either suffering or being in pain, and the Warden's kindness puts a damper on her good times.
These are only inferences based on the dialogue you get when you make certain choices (She berates you for offering to help the Blacksmith, Mayor, and Knights in redcliffe very clearly).
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