It is safe to assume, that these creatures could very well pose a threat to Mankind if they're ever to become organized.
We managed to slay ~95% of their projected numbers, and have been working to keep lowering their numbers from then on.
So not only does a faction refer to itself as Mankind, but they're also large and powerful enough to destroy 95% of a galactic threat after they were distracted.
This is very much relevant to my interests.
Being distracted and gathered made them an easy target (nuclear launch detected).
I was watching a video when you posted this.
Also, there's a variety of factors. Think of the Slime as Orks, and Mankind as it's many splintered chapters of Space Marines.
The Slimes war against each other, making a big ol' noise, and Mankind decides to do something, and unite for one act...
Mass Exterminatus, one for every world infested with this slimy menace.