When I suggested for Hitler a possible defense of 'insanity', I was presuming the well-known British gift for fair-play, compromise...and ruthless self-interest.
After all, look at the way we dealt with Rudolph Hess!
OTL, in its latter years, we fought WWII with great disregard for (enemy) human life and - at its ATL ending - such an apparently enlightened judgement for the conflict's primary architect is nothing of the kind.
Firstly, it would have demonstrated to the entire German people that, far from following a military genius, they had (mis)placed their trust into the hands of a drooling madman - who had (mis)led them to humiliation...again.
That would have made post-war resurgance of die-hard Nazi-ism far less popular: who wants to admit they were fooled by a liar/megalomaniac (cf: OTL - PM Blair, over Iraq etc).
Secondly, it would be intended from the outset that the puspose of such a trial would be the humiliation of Hitler (and the destruction of his legend) and, inevitably, he would not be required to outlive it by very long.
I'm sure that Allied Intelligence had access to many traceless poisons!
It has been rumoured that one or more terminally-ill British momarchs were helped on their way by judicious overdoses of opiates; how much less scruple then to dispose of a monster, once his show-trial had been milked to maximum effect?