>>714293162
As a general rule, the Inquisition has to be: one of the more radical ones in order to contemplate working with one of Mega-Satan's little helpers; be supported by a coterie strong enough to potentially take on a Rogue Space Marine who at best is just a transhuman living weapon who really wants to skin people like them alive and at worst is that but also all hepped up on Chaos Juice; and be facing a crisis so dire that endangering themselves, their soul, and their mission by working closely with a Chaos Marine is the only option, even if it leads to a more puritanical Inquisitor blamming them afterwards. On the Chaos Marine side, the Night Lord has to be: so incredibly desparate that they have no other option than to side with servants of the Corpse Emperor - no other chaos warbands, no cultists in need of muscle / leadership, no targets to enslave or terrify; one of the "Chaos has no hold on me, I'm a free agent who just really likes bats"; potentially trying to tempt the Inquisitor to the Dark Side, although that's not really the Night Lord's schtick.

TLDR: on one side you need an idiot, and on the other a loser.