Emps was kinda of a dick eh?

Yeah, Magnus's fall was kind of the Emperor's fault.

Magnus: Can you tell me why I can't use my psychic powers?
Emperor: No
Magnus: Why?
Emperor: Because...... I have my reasons......
Magnus:....

Oh yeah thats true, they did kind of stop being cute little ether pets and were like "yo we are smart bte and we can offer you this power" at the end, forgot about that. Ok good point

When the time was right they offered the sons a lot of power and more or less turned them to chaos. I'd say they were demons.

i dont know if id consider the Tutelary's demons, if thats what you are refering to

Let's be fair here. The Emperor was working on a plan to enter the Eldar webways, and he didn't need or want anyone messing with the Warp. Also, Magnus and all his followers were corrupt as fuck because they all had little demons following them around. And finally, despite them using many awesome psycher abilities at the battle for their homeworld, they still got their asses kicked by the Wolves so what good was it? The only thing they managed to do was destroy the emperor's plan to enter the webways BY ACCIDENT while in the warp. So yeah. The emperor was right, Magnus and his ilk were foolish traitors who didn't trust the supremest psycher in the universe.