But lanius hates the legion. Hes only dedicated to Ceaser himself. So if Ceaser died Lanius would probably just leave the legion or something else.
Strange, he stayed when I killed Caesar and had him win.
He lives for blood, he lives for murder, he lives... For battle...
Legion can provide that.
And making "him" out of all the people the de facto "leader" would allow him to steer the Legion the direction he wants it.
And from the ending sliders, he uses it for brutal warfare.