There are no good religious options in the Imperium but I will allow that every single one of them is better than "I will bargain with a C'tan shard because I am sure that Mr. Nightbringer The Herald of Sorrow will be grateful and cut me a square deal."
Theres not any excuse of ignorance either. He was working with dark elves. He had a copy of the necron codex. He knew that C'tan shards name and intentions. It would make more sense to join a genestealer cult.
"Governor, the fact that Khorne's own freight train is using our war crimes memorial to offload its passengers is troubling and I fear its no coincidence."
"Superstitious nonsense, we put up no trespassing signs on the pile of skulls."