The thing is though, sitting on the golden throne is not a reward, or in anyway a good thing. Its damnation. Constant agony that slowly kills you. Look what its doing to the emperor, who is much more powerful than magnus.

Perhaps the emperor saw a future where he told magnus everything and magnus rejected a life of torture and pain that would eventually kill him and turned to chaos sooner.

Magnus fucked up but he was loyal, right until the last moment, even when the wolves had wiped out nearly all of prospero. Maybe the emperor put hus faith in the fact that magnus was one of the more loyal sons, even though he was arrogant and naive. (Like how many primarchs can you say arent arrogant after all? Only a few)

At the end of the day we know everything about magnuses perspective, but very little about the emperors, so without a complete picture we cant really make the judgment that the emperor was wrong to not tell magnus like he did. For all we know, that was the safest option. Could well of been a damned if you do, damned if you dont!