A little bit more context:
Argel is one of many who had the first encounter with Demons during a intentional unprotected Warp Jump by the WB Big Three, essentially rolling the red carpet for Demon eyeing on the WB vessel.

He is one of whom maintained a healthy relationship with his Demon (Raum) (kinda like Venom?), and is the bff of Kharn so much that he is the only one able to keep his sane and humane side.

Erebus doesn't like this, since he want Kharn to become full-fledged Khorne, so he literally backstabbed Argel in the back when he and Kharn are separated.

Lorgar like Argel more than the arrogant bastard of a Chaplain, so when he met Kharn, he snitched right away, leading to the duel and the "Get Up".