Spike didn't see Xander as a worthy adversary. In fact, he didn't view Xander as much of anything beyond a minor annoyance and, if he wasn't involved with the Scoobies, would have throttled him for the sheer hell of it.
Where Xander saw an intricate conspiracy out to get him, the reality was just a bored vampire looking for a momentary diversion.
Xander just didn't get the nature of game.
spikedawn — challenge #8: nature
open_on_sunday — challenge #110: enemies
Part of the Mischief!verse