Has the void ever mounted a large scale attack on the Source on it's own?
See lots of cases throughout history where they were summoned to attack. Or people on this side made deals to bring them. But as far as I know they lack the power to open portals and come over on their own. The Source has to initiate contact.
In which case...
Are we justified in going over there, killing what is going to end up being a shit ton of them along with their leaders, only to rescue a single POW of an alien race that does not even belong on our own world in the first place?
The void is a bad place. Not going to defend that. But it is THEIR place.