A little insult can go a long way

Janitor is the best evil genius

See I think the reason why that works is because you can't argue that really. I mean you will go over it in your head how am I stupid in his case he probably go over in his head all his school accomplishments all his grades all his school awards and think I did all that how am I stupid. And it will eat at him. But if he was told why he was stupid he have something to argue like if he said he was stupid because he couldn't tie his shoe he be able to look up if other smart people couldn't. It works the same when you call someone ugly and they will question what part of them makes them ugly. But if you said you have an ugly nose they could argue it.