And even if objectively it is a mistake on the same level if Dean himself does not see it that way then it doesn't mean much in terms of his ability to relate to Sam and his mistakes prior to his recent decisions.
Well, but if Dean does not see almost causing the apocalypse because he could not let his brother go as a mistake, then he will never see whatever damage Ezekiel may inflict on the world because he could not let Sam go as a mistake either. ;) In both scenarions the fact that Sam lives to see another day will always win out over Dean's guilty conscience then.