Well, I think you;re assuming that fans know as much about publishing as writers do, and this is seldom the case. Many fans care passionately about books and characters, and have no clue that writers don't get to make many of the decisions about what to write, what gets published, and what doesn't.
They don't have a clue why book three never appeared, and assume it's because the writer decided not to write it.
I don't think many of them are unbalanced, they're just passionate, and this is usually a good thing.
Unless a fan really shows signs of true insanity, I try to interact with all of them, and it usually works out very well. If you think something like book three not appearing can make fans angry, try refusing any interaction. Now you have a fan who's angry because of book three, and who is now also angry because you're too snooty to reply to him.
Most fans just want to know why book three was never released, and I think the writer owes them this much. Fans are usually good people, even when they get angry, and a simple explanation can save a fan, and even cause fans to write enough letters to your publisher to make the bean counters change their mind about book three.