The mere fact that it is possible to 'recover' a Borg (i.e. remove it from the Collective and return it to being an individual, such as Seven of Nine) is all the proof I need: Genocide against the Borg is no different than genocide against any living race. If assimilation were absolutely irrecoverable, then there would be less of a problem with wiping out the Borg. But this is clearly not the case.
And even if the Borg don't respect the rules of war, that doesn't mean the Federation shouldn't.