Neville Chamberlain wanted to negotiate and try diplomacy with the person who caused the Holocaust.

I think that there's a problem with your analogy here.  Correction: not a problem, but something to  watch out for.  I don't know enough about Chamberlain to comment on his actions, but given his position prior to the War I don't think it should be outright criticized as the wrong way of going about things.  We didn't know the true extent of Nazi atrocities until after the War was over, so we cannot really comment on Chamberlain's actions with our knowledge, which is truly after-the-fact.I don't think it's wrong for leaders to search for diplomatic solutions to problems to avoid very large problems down the road.  As a matter of fact, I think it's the truly good thing to do.  As for Iran in particular, I think that the key is that if the country wants to enter the modern era and the economic advantages that come from it, it will have to come to the negotiating table and comply with the rules of the international community.  If Iran wants to keep threatening, then investors will stay away and Iran will remain in the dark age - just as North Korea has done to itself.

The question becomes this – what do we do about it?  Also, what do people consider to be a “threat”?  The Soviet Union was a “threat”, yet we were able to deal with it without resorting to war.  Cuba was a “threat” as well, and we dealt with it with a certain policy.  Iran, too, is a “threat”, but it's not too unlike others we have dealt with in the past.

Really, and unfortunatly, all that can be done is let him talk. Until there is proof that he is going to do something, or already has, then you have to let him talk. But that is a two way street.