I'm going to try something rather novel for me... being an Emperor optimist.
If we are dealing with an Emperor who is basically his Dune inspiration, Leto the Second, the Worm, someone who genuinely thought that he had only one path and he was the one who had to walk it then he could be a genuinely good person. I do not mean 'by the standards of the Imperium' or 'considering...' I mean full stop he could have been a good person who did unspeakable things. Logically if you can see that the only path to avoiding worse harm is you being a monster then being a monster if the right thing to do, it is the rationalization of every tyrant in the history of humanity, only in this case it would be the objective truth because he can literally see the future.
Now comes the hedging, 40K isn't actually Dune and divination does not work like that, it is never certain which means the Emperor was a monster based not on certainty, but on his best guess. That turns him from the genuine hero that was Leto the Second into a tragic figure undone by his own pride.
Thing is if that is the case... it is something that offering him some good unbiased counsel might actually help with. Fan can make a reasonable claim to being his peer so he might be willing to listen.