I would agree that leaving his 'grudge' out of the note would have been necessary since it undoubtedly would identify him. Its like the guy who went around vandalizing cars that did not have a front license plate and leaving notes signed by "Committee for something or other". The cops were no fools, they knew there was no committee, just one nutcase somewhere and all they did was ask the newspapers to search their letters to the editors reject pile.

For this reason, I think the 'grudge' related to something a bit more than letting a door slam on him. That would not be identifying. Something more precise in John or Patsy's past. Something. But what?