"John himself" implies that it was less expected that John would do so, and in this usage is synonymous with the word "even", as in, "Even John did so."
John is a motivational speaker who always encourages people to love themselves and will never declare that somebody authentically hates themselves; instead, he will always encourage them to realise that they do not actually hate themselves, deep inside. Still, people were skeptical even John could have this optimistic attitude when he met Gregor, an unusually self-loathing person. After their encounter, the participants in the seminar were enthused that, as expected, pretty much every person there had felt that Gregor truly did hate himself and would never budge in his views towards himself - even the speaker. John himself said that he hates himself.