Agree with AmbientD2. If she had gone postal on the couple who owned the flat, or on the real estate developers who profited from the common people's misery, or the health insurance executives who were ultimately responsible for denying her father's healthcare, then she would have been somewhat a sympathetic character.
But she killed complete strangers who had done no wrong to her, even an unborn baby. I was rooting against her too, everytime one of her victims wounded her, and I ended up despising her. Sure, she suffered a lot of injustices all throughout her life. But because she was willing to inflict the same kind of injustices upon other innocent people, she lost the right to complain about the injustices done to her. She proved herself just as base as the real estate developers, the insurance company, and the greedy couple who owned the flat.
Still, it was a great movie. Did anyone notice that she probably hadn't initially planned to take out the people in the flat above the one she wanted? If they had kept their stereo down, she probably wouldn't have gone up there. After she got done killing the neighbors to the elderly couple's flat, she heard the blasting stereo, and probably thought that if she moved into the elderly couple's flat, she didn't want noisy neighbors upstairs (like the noisy neighbors at her parent's flat). Also, I don't think she would have gotten away with it, because when she left the apartment upstairs, she stepped all through the blood on the floor (+ her bleeding ankle), and would have left a blood trail exiting the apartment, which wouldn't make sense. The CSI guys would have realized this.
"No *beep* sh*t lady, do I sound like I'm ordering a pizza?" - John McClane
..I'll get my cape...