If someone is honestly mistaken about something the Bible teaches, and they are telling others what they think it says, are they a false teacher? It is true that they would be spreading a false teaching, but that doesn't necessarily make them a false teacher.
So what makes a false teacher a false teacher? See Am I Really a False Teacher? by George Sarris.
The man in the video below may fit the category of a false teacher. I can't see how he can honestly come to some of his conclusions. He has some nutty views and I don't agree with many of the things he says. His message in the following video is abrasive. It's so abrasive that I wasn't going to share it on this site. But I decided to because on this particular issue he makes some very important points that Christians ought to carefully consider. He certainly sums up what many people think (see the comments on YouTube).
What he says about this doctrine causing more people to reject God than all the wars and all the oppressive regimes combined may have some truth to it. Japan might have become a predominantly Christian country if it was not for that doctrine (see the video below). Not only has the doctrine caused many to doubt God's goodness, it has been the primary justification for persecution of heretics and Christians by other "Christians." See "A Legacy of Fear and Persecution" in Thomas Talbott's book The Inescapable Love of God.
Nowhere in the Bible does it say that a person has to believe the doctrine of eternal torment to be a Christian. It reflects badly on the character of God and has turned many people away from Him. If you agree with those two statements you might find the following interesting.