I think that Spikes role was going to be to kill Buffy. The First triggers him, and he goes wacko, killing off Buffy and the Scoobies. He was the most likely to succeed, seeing as how he killed two slayers previously.
HOWEVER, the trigger seemed to dull his intelligence, as freakin' PRINCIPAL WOOD kicked his ass untill he snapped out of it.
And I think the 'eliminate the slayers' plotline continued to almost the last episode. While it wasn't the Firs'ts primary goal anymore, it did wipe out a lot of the potentials. We just don't know how many.