User blog comment:SniperGhost/Gulliman's Return/@comment-25033299-20111021154923/@comment-1932673-20111021184545

All souls go to the warp when a sentient perishes, apart from those of craftworld eldar and the primarchs were created using forbidden knowledge, which GW has hinted was gained from the ruinous powers. So its not that far fetched, and considering how ridiculed he was by the end, Gulliman would probally have become bitter or at the least have some sort of chink in his moral armour.