In my point of view all IG officers must right now be much more relaxed than before, the "psycho high commander officer killer" dead all could breath again.
The moral impact would be pretty low because of the fact the man was hated by his own troops. New leadership could only mean better life for all IG in the Salient.
Don't forget also that Tetrachus the crusade leader is right now quite pleased by the loss of this mad officer.
The murder had been made in private then no one outside the room know the truth, so they can make up all story they need to cover the real fact. In an other salient the lord commander is dead years ago and the officers are leading the salient under the cover of the late high commander name.
But in the regard of the deathwatch what does it mean?
First, one SM had been a little rash and act with a behaviour that can be suspect so they will test him for his moral corruption (then reknown loss, because brothers of the DW will know).
Secondly, the player act against his own squad so there will be cohesion loss untill the confidence between the SM is restored.
Thirdly, congratulation brother a big pain in the ass of the DW in the salient lay dead. Great! We can not promote you but thank you none the less.
At last, depending who get the high commander dead boots maybe the DW have now a special relationship between the salient forces and the DW. And for the rest of the crusade commanding officers, they will know that you don't have to mess around SM.
For the fact in legion of the damned novel, the SM hero kill a lot of high ranking members of Ecclesiarchy without any problem because he was thinking about the numbers of frater militia deployed and accussed the ecclesiarchy to break I don't remember witch edict forbidding the church to get an army under command. The emperor angels are upon the law. And more, they make their law.