So anyone can just ask any opinion based question here about plot lines and stuff.

I have a question, actually. If a character who had been teased to possibly arrive throughout the entirety of the book FINALLY does arrive, would it be a horrible idea to kill them off? Like, would it anger you as the reader?

They're important to the MC and their death will change the outcome of their lives for the better (kinda), but that character will literally be teased for like 85% of the book and only be there for like 5%. Bad idea? Good idea?