That was the plan; Faith was only meant to be in 5 episodes, she's the slayer who gets too caught up in the fun of slaying and, after she accidentally kills Deputy Mayor- takes her own life.
The plot would've highlighted the perils that come from the dark power of the slayer and the instability that comes with it.
But Eliza- only 17 at the time- impressed Joss greatly so he changed things around a bit (this is why she isn't present in some episodes for some reason- 'Band Candy', 'Gingerbread', Helpless') and made her corrupted by the act instead.
He also added a lot of more specific dialogue to her scenes with the Mayor in Choices and GD; making that relationship more human/sympathetic and a parallel with Buffy/Giles. Its actually remarkably that it worked out as well as it did.
