I think I could write a book on the subject lol but I'll try to limit myself. Basicly, no I don't think the gang treated her badly. I think more could have been done to help her out, but she had issues and it takes more than a few kids to solve that. Faith was self-destructive, lost and plainly out of control. Buffy, Xander and Willow could not change that for her, they weren't trained to deal with Faith's mental state.
Faith reminds me a lot of myself in a lot of ways, and at that point, I don't think any of the Scoobies could have prevented her from going down the path she went. Underneath it all, I truly do think that she was reaching out for help, but she wasn't ready to accept it, so it really was pretty pointless. That's why she turned to the Mayor, he could offer him something that no one else could: structure. At that point, although Buffy was her friend, it was not exactly what she needed, she needed someone to help her control her emotions, someone to teach her right and wrong. Buffy, Xander and Willow couldn't do that. So in the end I somewhat do agree with Xander that Wesley should have done more. She needed a parental figure. There's a reason kids her age don't usually live on their own, they are simply not ready. As much as they can say that they are, the structure of the brain proves that they aren't. At the time, the only adult in charge of her was Wesley, so by association he should have been the one to put his step down and act as the "father" figure. But again look at Wesley in s3, he was definitely not ready to deal with that. One of the reason that she ended up confiding in Angel is exactly because he wasn't her friend, but at the same time he was able to help her, he could teach her to control the beast in her.
I'd also like to add that I also think that Faith panicked when her and Buffy became close simply because she didn't know what love was. Faith doesn't understand closeness, as shown with Xander and the whole choking scene. With Buffy, it was probably the closest she's come to loving someone, and I don't think she understood how to deal with that. Killing someone did freak her out, and I think part of it was because she knew people would now look down on her. She was starting to have a family and I think she was scared of losing that, which prompted her to become self-destructive, since it's easier to accept that you decided to lose people than to lose them for reasons out of your control. Faith hated not being in control, and when she felt that she was losing her grip on her life, she panicked. And again, that started when Buffy started showing her love, which Faith had never experienced.
Another aspect is her Watcher that was killed before she came to Sunnydale. I talked earlier about how Faith needed structure in her life. Well losing her first watcher broke that structure. We didn't get to see her before when he was still around, but I'm sure him dying did send her closer to insanity. All in all, I think Faith was just incredibly unlucky, and everything that shouldn't have happened happened. Had she never killed that man or had her first watcher never died, I think there was a good chance that she would have made it through and stayed "good".