Testing the weapon on the target really is "out there." In retrospect, I can see why they did that storywise - to show how big a threat the Xindi were. The played the Death Star card. Okay, I can live with that - but, yeah, when you get into the Xindi just believing whatever the Founders - I mean Sphere Builders
- tell them, it does get a bit "out there."
To me, it would have worked better if we learned that they were out to destroy us to prevent their own destruction in the future, because they had encountered a probe or message buoy from the future *of their own* telling them about us in the future and a coming conflict that resulted in their virtual extermination. Turn Earth into a *real* threat, not an imagined one, and work within the TCW arc so that our heroes have to both avert the creation of the planet killer and
change the future. The whole Sphere Builder subplot strikes me as an added layer that didn't need to happen.