No one thinks diet pop is good for you...but bad? Everything is bad at some point when consumed in excess. Even water. Diet soda is no more harmful than regular soda, actually better in that you don't get a ton of empty calories from it.
As far as aspartame goes, it isn't bad for you. Well, unless you have the rare genetic disorder phenylketonuria. Aspartame is made up of two amino acids... the same ones that are found naturally in certain foods. For some reason people are just ideologically opposed to the idea of aspartame and want to blame everything on it and insist that it is some sort of poison.
The internet is full of misinformation.