Simple. In one case we’re all together inside our fortified city, and the evil enemy we all recognize is outside the gates, ready to attack with her hordes. In the other case we’re all together inside our fortified city, and an evil enemy — supposedly one of our own that some mistakenly think is a friend — is within, organizing his supporters to come at us from behind as we focus on the enemy outside.
The real argument here is whether or not Giuliani is an enemy to our cause(s). I firmly believe that he is; I suspect that you feel differently. But if he is an enemy to our cause, it’s obvious that an enemy within is more dangerous than an enemy outside the gates — just as civil wars are much more more bloody and dangerous than even huge wars fought against outside forces.
If you want to use military metaphors, wouldn’t we want as many people on our side as possible?
If you want to insist Rudy’s a traitor in our midst, fine; you’ve picked a side. I just can’t promise you it’ll be the victorious one.