I think the idea already brought up about terrorizing and demoralizing the enemy with a first strike from an unknown origin, before delivering the final blow, makes sense too. Why waste all that thirst for vengeance on an instantaneous, virtually painless genocide, when you could also terrorize and torture the enemy, make them suffer, fill them with dread and paranoia?
Very illogical if the purpose of destroying earth wasn't about vengeance but about protecting their own world from being destroyed. It just seems odd they would have so much bloodlust over something that hasn't actually happened yet.
I blame it on the Reptilians. They were probably thrilled to be told that there was a future need for vengeance that they could do right now! In fact I bet the sphere builders tweaked how they told the story of the future destruction of the Xindi according to what race they were talking to.