Faith wasn't the first Slayer to choose the wrong path, whatever the Council would have people believe. Far from it.
Some sold their talents to the highest bidder, with no consideration for the winner's morals. Others, driven by fear or an overriding will to endure, even if it couldn't be termed "survival", made deals with demons in return for protection, or even with master vampires to be turned. The temptation of the dark is strong.
When such things happened, the Council devoted all its vast resources to... dealing with the problem. Even they strayed from the light when they must.