The carol wasn't useful because it had Harumaki in it and there's no set number of mafia included in the carol. With that being said, anyone left could have been mafia with Harumaki and lynching outside of confirmed is perfectly fair. He misread the carol (and participated sufficiently before that) and lynched based on his incorrect reading. I don't think it's worthy of a violation based on the fact that:
#1 it was a small, yet honest mistake #2 he had no intention to lose and had participated sufficiently before that