Oliver Kamm provides some much needed clarity as to why Iran's Holocaust Denial conference is not merely offensive but a crime against historical truth :

What is wrong with the Iranian conference is thus not that it's offensive, but that it's a fraudulent gathering designed to generate hatred through lies. It's an offence not against our feelings but against truth and against history. The problem with the puppet-President of Iran is not only (or at all) that he is ill-mannered: it is that he's a bigot, a racist and a messianic crank. The proper task for Western diplomacy is to say so.

Kamm's simple explanation stands in apposition to the Guardian's explanation which seems to accept the premise that denying the Holocaust is offensive (to Jews) the way the Danish Mohammed cartoons were offensive to Muslims. It appears that the Guardian regards this conference as Muslim retaliation to being insulted, admittedly they say, a misguided retaliation, rooted, so they claim, in Ahmadinejad's "misunderstanding of how western democracies function".

As Kamm point out, the Guardian rates this event as an "unpleasant episode", which downgrades it considerably from the perversion of norm, common sense, knowledge and truth that it is.