Yes 18 legions on a world the size of our sun. They have no ships to hit the world from above. This is mass warfare on a plant ran by the Flood. The legions mission is to wipe out everything on the planet. Do you think they can win?

I don't think the Flood could handle the plot armor 18 legions possess altogether.

Well we know one thing for certain, the sheer number of dead from the conflict would stain the world red for all time, and the mountains of corpses would pile to the heavens.

the flood can and would take over the dead SM. Its a fight from all sides. It would truly be chaos in the flesh.

I know nothing of the flood. But the dead are Nurgle's property.

Sgt. Johnson was a "Spartan 1". His neural pathways were a bit screwed up because of the experimentations, so now the flood could not infect him.
I am not sure how strong a Space Marine's defenses are.

Who knows, maybe the warp taint could actually render the flood unable to control nervous systems due to it being screwed over horrendously due to warp exposer. So I think a no flood infection form taking over a dead or alive chaos marine. Regular marines on the other hand...

Robot Jones gives no fucks for your fire.

Not even mad.

forever alone...

No, not really.