Goddess Kali is considered a force for good, not evil. She is a servant of the Supreme, doing the dirty work of killing evil. She kills the demoniac. Oddly enough often demoniac people worship her, thinking they will get powers. Ultimately she destroys them, though.
Well if all the evil that surrounds us that we bitch about daily on this thread is any indication, the blue being set on killing evil is a dismal failure as a GOD DESS.