So what is the counter argument? Enough people stand on both sides that there must be some logic on each side or is purely meme fodder.
Basically:
the Emperor is of course at fault: he treated the primarchs as tools, giving them orders without bothering to explain the reasons behind them
And sometimes those order went against the very essence of the primarchs... in this specific case, the emperor forbid the use of psychic powers, and Magnus was engineered made to use psychic powers. It would have been nice to give Magnus some hints of the danger posed by the Chaos Gods but nope "Do as i say because i say so"
That said, Magnus went totally out of line. The straw that broke the camel's back (as said by
@ChapterAquila92) was the moment when Magnus discovered that Horus was going to betray the emperor, and decided to warn the emperor by:
0 - (before such discovery he and his whole legion continued the researches on psychic powers)
1 - use illegal psychic powers
2 - ignore the order to "do not disturb me for any reason"
3 - instead of give a warning to friendly primarchs (for example, Sanguinius, that was on his side to support the use of the psy powers) he decided to go straight for the emperor
4 - he sensed the barrier around the Emperor's palace, and decided to break it with brute psychic force
5 - he disregarded all the advices by his most talended psikers to DON'T DO THAT
His intentions were not bad but he screwed hard, multiple times.