I guess this shows the "PTB" do not have a choice on who gets the slayer attributes...🙂
Actually, what makes you say that.
For all we know, the PTB's 'turned on' Faith to save her.
Remember who it is that saved her...Angel.
If she never turned to Slayer, she'd never have gone to Sunnydale, thus meeting Angel. This inturn led her to LA(eventually) where Angel put her on the path to redemption.
I think Faith was always destined to visit her darness born from the stuff in her life.
The killing of the Deputy was the catalyst, but not the cause, it was the subsequient treatment of the situation.
Buffy was forcing Faith when she clearly wasn't willing or able to do it, then when she told someone, she was found out a liar.
She did wrong, she knew it, but she focussed on her lack of support, this is what pushed her over the edge.
If she hadn't accidentally killed the deputy, she would have found another situation where she felt like an unsupported outsider.
Further proof of this is when Angel nearly gets through to her, but Wesley and the council charge her.
Angel is giving her the support that she needs, she is almost on the brink being on the good side, but that is taken away.
Thats the reason as well she falls in with the Mayor, and even as far as season 7, still looks fondly on him, in her own way, as he gave her love and support.....just the extra stuff he required that was bad.