It would only take a single Chaos Space Marine, armed with standard equipment, to rape... The Empire, Kislev, the Dwarves, the Orcs and Goblins, the Tomb King armies, the Vampire Count armies, Cathay, Albion, Ulthuan, Norse Tribes, Beastmen, and any other faction or power you can think of. And if I'm not mistaken CSMs Bolters use magical warp blessed rounds that in some cases materialize out of the Warp so no running out of ammo and if he did run out of ammo it's simple, he could use his fists, teeth, and other parts of his body to finish the rest off.
Chaos Space Marines in WHFB is the worst idea I heard, in terms of advancing a story or mod, as the whole Cain killing Abel episode. I mean did he really think he was gonna get away with it?