My issue was never really about a player killing another players character. As has been pointed out, you really don't need the Perils of the Warp to make an excuse to do that. Allthough it certainly helps ![]()
Personally, I feel that is it the duty of players and GM alike to help shape the story and the personalities of the PCs in such a way as to minimize such inter-party conflict. Tension is good, it adds to the roleplaying scene. And in some cases killing a fellow PC can be the right thing to do. Even fun for all parts involved.
But that is another discussion.
My issue was always with those who somehow think they have some natural authority over the psycher in the group, and who used blatantly meta-game information and out-of-character knowledge to justify actions that were totally out of place. In my not so humble opinion.
This is not the Imperial Guard! The Guard is a blunt instrument, and they can ill afford to issue complex instructions to their grunts. No Guardsman is ever going to make a competent and informed decision on the state of a psycher, so better to err on the side of caution. The guard has numbers on their side anyway, and can always requisition new psychers.
The Inquisition, on the other hand, is the scalpel, and it's servants are expected to show more skill, intelligence and sound judgment than those of the guard. The Guards standing order to terminate psychers "acting odd" does not apply, and their own Inquisitors will is paramount. While the acolytes might not like it, and may prefer the less dangerous route of avoiding or eliminating a psycher, who are they to question the decisions of their Inquisitor?
He recruited the psycher, he tasked him to the mission. He knows the risks, the dangers and fallout, yet he made that choice. The other poor sods in the unit will just have to put on a stiff upper lip and deal with it. Part of the job description, really.