His political beliefs.
He’s so stupid, he didn’t even know what that symbol meant, according to what I’ve read; and he was puzzled by the backlash. (I think his brother and SIL tried to talk him out of it.)
I remember when Harry wore that Nazi uniform to a party when he was a teenager.
The theme of the party was, “Wear the most offensive costume you can think of” or something to that effect.
Harry must have won the grand prize.