Most evil people are perfectly convinced, that they are good.
I think it is okay, that people does not have alignment, I always found, that the alignment system in BG does not make sense, in many cases, at the end you only do what you want..
Unless you have a whole system organized, that changes your alignment, like in planescape torment. If you act evil your alignment changes to evil, if you make jokes, your alignment changes to chaotic,
but I think that would be overkill to implement.
