"... if opponents of all important truths do not exist, it is indispensable to imagine them, and supply them with the strongest arguments which the most skilful devil's advocate can conjure up".
John Stuart Mill. On Liberty.
I doubt if even the most ingenious devil's advocate could have dreamt up Holocaust denial, but even if we allow Mill's utilitarian argument to stand and to assert that the absolute right to free speech is an essential guarantee against tyranny, the Oxford Union should be ashamed of itself.
Every right imposes on others the duty to respect it. In the case of free speech, the duty imposed is not to prosecute or persecute those who express opinions other than yours. It does not impose a duty to help those who would spread lies, to spread them. It does not impose a duty to help publish the views of those that express hatred of others. It does not impose a duty to allow those that would deny the right of free speech to pose as the victims of censorship. It does not impose a duty to legitimate fascism.
However, Mill's essay does impose another duty; to stand up for truth, to vigorously oppose falsehood, even if it is not to be suppressed. The Oxford Union have failed in that duty, and those who, like Max Hastings, think that "the debate can do no harm" should remember another, somewhat contradictory, sentence from Mill's essay.
"But, indeed, the dictum that truth always triumphs over persecution, is one of those pleasant falsehoods which men repeat after one another till they pass into commonplaces, but which all experience refutes".