This question has been debated since the character made her debut as one of Batman’s villains. Is she a bad kitty or isn’t she? Perhaps she’s a good girl at heart who is just frustrated with the world and has become disillusioned with that which all consider righteous. Then again, she’s done some pretty nasty things in her day. Yet through it all she still admitted to having a conscience, something many evil doers lack, especially when dealing with Batman’s evil enemies. "Life would be so much easier if I didn't have this conscience!" she said in Catwoman Vol. 1 #9.
Catwoman always knows what she wants. She plays for her own side, instead of being a part of such defining things as good or evil. That seems far too black and white for her. She serves her own purpose and becomes allies with whomever she requires for the moment. She has no allegiance but her own and that seems to work pretty well for her.
Evil is such a harsh word to pin somebody with though. Could it be that she is good, but just likes the thrill of stealing? Does being a thief, and a darn good thief, make her inherently evil? She doesn’t seem to be too wrapped up in taking hostages, or blowing things up, or actually even hurting anyone who doesn’t cross her first. In fact, compared to Batman’s other enemies, she seems pretty low key with the pyrotechnics and innocent kidnappings.
She’s obviously considered one of Batman’s enemies, not because she is evil, but because he just doesn’t like her taking things that do not belong to her in the first place. And as most felines go, she does not like being told what to do. If she wants to steal, she’s going to steal and nobody, not even Batman, will tell her otherwise.
Therefore, Catwoman seems to be on the side of revenge and vengeance, rather than good or evil. Step on her tail and she will punish you. Rub her the wrong way, and you will know it. Betray her and... just don't betray her. Always remember the proverb from The Mourning Bride by William Congreve, “Hell hath no fury like a woman scorned.”
Catwoman first appeared, of course, in 1940's Batman #1.