That was back when the sword was still contracted to Genocide dude. And it did end up messing with the new user's head, so that was a valid precaution. But if they'd kill the genocidal guy first, Yura could form a new contract with the genocide blade. Or at the very least, by killing the guy, all of the contracted blade users would die, meaning they could use all the swords now and have massive firepower. So the question stands. If they feel like they can take out the enemy HQ (and without their only sword user to boot, so they're quite confident), why didn't they just do that before?
