i disagree. Luther saw the imperium for what it was. a massive galaxy pimp. whoreing out worlds it enslaves to use them for all their resoures before casting them to the shadows. Thats why he turned on the imperium, he was loyal to his planet and people and did not want to see his homeworld raped of its identity.
also, the reason why i think Jonson distanced himself from Luther was that Jonson knew that Luther contemplated letting the bomb rigged ship explode at the end of Decent of Angles, thus killing him. even though Luther didnt, Lion knew he considered it. so he banished him (and his men) back to Caliban until he could decide how to deal with his (up until then) most trusted friend.
What got me with that, is why he sent Zal back as well. I figure it is because Johnson was afraid of the power that Zal and Ist have being librarians. And the battle which took part just before scared Johnson into believing that they were more powerful then him.
But then there was the part in the briefing room where Ist spoke about the phsyic bomb and Johnson was confused and hated being over shadowed. But again, why send back Zal, as he was argueablly one of the best warriors under Johnsons command.
I don't want any people to ruin it for me if it is cleared up in FOA as I'm about 80 pages in on it

... but from the sounds of it... with people being left to guess... I can safely assume the issue wasn't cleared up in the novel.
I just hate how they protray Johnson as a giant Dick... just like... I don't like you, so leave...