...I'm trying to square your assessment of him with this:
From the wiki, to be clear. It goes on to state that while Horus certainly believed Roboute to be envious, it only Horus's belief.
I'm also just...not seeing any reference to the rebellion of which you speak. At all. In fact, the wiki claims that Roboute was in fact dutifully amassing a massive force on Calth at Horus's order when the Heresy began, part of Horus's schemes to weaken Roboute's forces at the start of the Heresy.
Where are you getting all of this? Is there some source the wiki blacklisted which reveals the dark truth of Guilliman, or something?