Magnus' actions not being understood or believed, while attempting to warn the Emperor of the danger of rebellion, is already a pathos-filled tragedy... he's 40K's Cassandra.

Magnus' actions in wiping out most of the Astropaths on Terra when it was already too late just makes him look like a bumbling buffoon. He might as well finish up by saying "Uh-oh, spaghettios!" with a canned laughter track appended, then slipping over on a banana skin accompanied by a comedy whistle sound.

His tragedy is supposed to arise from thinking himself the equal or better of the Emperor, but being confronted by his own inadequacies... it's one of hubris and pride and arrogance and naivete. There's no tragedy in just being flat-out stupid.