Archer had a Weapon of Mass Destruction with which he could lay waste to planet after planet until the Xindi were all dead... He had plans enough to kill thousands of Xindi at the Kemocite processing plant and thousands more at the "weapon" installation factory on Azati Prime... But that would still leave dozens of Xindi worlds with not only the surviving infrastructure to build a new WMD death star, but also a good enough reason to launch a conventional attack and give up on a painless humane solution to the Humanity question.
If he couldn't make friends with the Xindi, leaving any of them alive was a tactical nightmare.
If he couldn't make friends with the Xindi, leaving any of them alive was a tactical nightmare.