User blog comment:SniperGhost/Gulliman's Return/@comment-25033299-20111021154923/@comment-3258762-20111021184404

Exactly. It took a tainted weapon and a near-death state with lots of priests and sacrifises to bring Horus down, Guilliman died from the Serathi and was purer than Horus in my opinion. I think he if anyone would be the least likely to turn.