What made her change all of a sudden? Was she good all along and just having fun and got Batman beat up for no reason? If she was a hero why was she still convincing Batman to leave the people and run away with her? If she cared about those people why was she running around saving kids from bullies?
You apparently do not understand what character development means. It is when one character changes in some way over the course of the story. So no, she was not "good all along" as that would imply that she didn't change her character but rather hid it in some way.
She changed because even though she was acting out of self interest, she had a concious. Hence why she took in that girlfriend of hers as a partner or apprentice or whatever and why she only stole from the rich rather than from the needy and why she felt guilty and horrified at what Bane did to Batman, knowing it was her fault. But she buried her concious under the rationalization of self survival, that taking from the rich was okay because she needed it (inconsiderate of what that necklace might mean to Bruce, for example) and that nearly killing Batman made her life better.
Batman, however, sets a higher standard. He is truly willing to give it all, and even after all that, placed his trust in her to do the right thing. Realizing how much her actions impacted everyone else, she decided that pure self interest wasn't as easy or pleasant as she thought it was. I think it's clear that she had a lot of sleepless nights over what she did. Her discomfort when she sees Bruce return is palpable. So she comes in to save Batman. She sees that being a better person than what she is is possible, and becomes that.
Also why does she dress up as a cat? Does she just like the animals and thinks she looks cute in it?
Uh...she doesn't. She wears goggles to help her steal stuff, which happen to look like cat ears. Other than that and the bladed high heels, she wears a standard spy suit with a mask, and the papers have dubbed her the cat, most likely because of her occupation, a cat burgler.
Ok, I realize not all characters need development and have complex/unpredictable scenes, but it takes away from a movies that take themselves very seriously.
It would, if what your saying were true.
Catwomen was fantastically written and played in Batman Returns..I just thought Nolan was looking to one up here but left disappointed. Hatheway is mesmerizingly gorgeous though but she felt shoe-horned in this movie, sorry.
She was a major part of the story. Not just because she was the reason he got his back broken and then later saved his life, but also because she helped him reconnect to his connection to humanity. He trusted her and she came through. On top of that, she is the only woman who has seen his true face and actually embraced it. 99% of other women didn't know Bruce was a persona and Rachel rejected Bruce so long as he was Batman, so this makes her special as love interest too. Shoehorned is the furthest thing from accurately describing her.