While the source of the Buffy's/Faith's power may be demonic in nature(Read Tales of the Slayer--the comic one, the first story)the possibility does indeed exist that she may go bad. I mean the 1st Slayer may be a demon/human hybrid--working in reverse. This is my theory, of course. Giles said that the first true vampire--in order to stay alive--mixed his blood with a human, and became a demon. Is it possible that the evolution could've produce a another demon that killed vampires instead of making them into more vampires? Keep on watching "Restless," and just think about it. The 1st Slayer was pure monster...she was a lean, mean killing machine. She was incapable of any rationale thought. Buffy says "I walk. I talk. I shop, I sneeze. I'm gonna be a fireman when the floods roll back." And all the 1st Slayer says, "I am destruction. Absolute ." Remember the scene when Giles is being pursued by the 1st Slayer(when of many actually). She cuts his head open, and he goes, "of course, how could you have known(the difference between who to kill and not to) you never had a Watcher." It is possible that the Watcher position originated to control the Slayer from killing the wrong person. Remember Faith didn't have a Watcher, and looked what happened. Giles and Buffy even talked about it in "Consequences." Now, I am not theorizing a Watcher in the form of person, but in the form of a support group. In Buffy's case, it would be Giles--now the Scoobies(including Spike--remember his actions in Dead Things. He does everything he can, because he knows that she is important to the fight). We all saw what happened to Faith when she didn't have a Watcher. She went to the other side. However, she wasn't pure evil. She was still human, and it was she who told Buffy on how to defeat the Mayor. Furthermore, as much as was paid to, she didn't want to kill Angel. She only wanted to upset Angel enough so that she could get killed--effectively suicide by cop. Now, that she has a sponser--in a sense her Watcher--Angel(and the rest)she will be reformed(hopefully).
I just don't see an evil Faith returning mainly because the fans liked Faith as a "good" slayer in the first place, and Joss would really do a disservice to bring her back as an evil one. I know that I'll be mad!