"I don't care if you want to have fun at metal's expense, but if you brazenly mix up black metal and death metal – and the essence of the joke hinges on getting that reference right – well, that's just lazy writing and it deserves to be called out and mocked relentlessly. But that's totally pardonable compared to calling Judas Priest a death metal band."
Yikes! And Metal Insider chimed in as well,
"While it'd be ridiculous to suggest that Opeth, Amon Amarth or Entombed would be on the show in place of Priest, it might have been a good idea to not call a 43 year old song that gets played on classic rock 'death metal.'"
So The Simpsons apologized the only way they knew how, with Bart writing on the blackboard. As pictured above, Bart let all the Judas Priest fans know that The Simpsons were truly sorry and it would never happen again. All was forgiven and all is right in the world.
The Guardian has the story