The amusing train of logic :
A: I won't believe anything you show me you manipulative monster !
B: *Shows no context confirmation bias*
A: I believe this 100 percent with all my heart.
It would be better proof if there was an ending path where the emperor reveals his evil master plan and dominates the brain, but there isn't one ?
The reveal with entralling Stelmane is interesting because the Knights of the Shield is a secret front for the devil Gargauth so you could interpret stelmane as a devil cultist, or the emperor both
Larian really needs to explain emperor's motivations better