General Haloverse lore, not multiplayer. Does anybody know why the Covenant decided to annihiliate humanity?
With every other race they encountered they had a war, and once the species was subjugated, instead of being destroyed, they were given purple armour, and absorbed into the Covenant.
Except the alien race known as humans. For some reason the Hierarchs decided all human worlds must be glassed.
There would probably have been human groups on remote colonies who would have been willing to become part of the Covenant, and provide it with valuable support in return.
Sure, the Covenant are rough, with little regard for the indivdual. But the UNSC are just as bad. UNSC marine is just about the most dangerous job ever.
At least if you were a Covenant infantryman you might get an energy shield. Even safer, you could get lucky and command a Scarab!