I just read this. My brief thoughts:
Cynth recounts half a dozen times John lost his temper (at her or Julian and Sean) and one count of violence. It doesn't make him a monster - just a typical dad of his times (my own dad was from Toxteth).
Cynth comes across as very compliant - her inability to stand up to John, even when confronted with his infidelity, is quite remarkable. Her (and no doubt Julians inherited) inability to deal with issues and subsequent meekness does, to me, make John seem worse than maybe he really was. John could be a thoughtless, selfcentred arsehole, but he was a product of his times. His rise to sainthood probably makes his fall from race seem even more extreme.
On the man himself - during Beatlemania, when he was being told what to do, what to say, how to behave all was well. When that stopped he fell into a blackhole. Perhaps he was so drawn to Yoko because again he was being told what to do, what to say, how to behave.......
And now, I don't think boredom was at the root of Johns post-touring dropout. He just wasn't fit for purpose. Paul was the man with the ideas (Sgt Pepper, MMT, Let it Be) and increasingly the tunes. With no one to tell him what to do he did nothing. He was a great rock and roller, but lacked the vision of Brian and Paul.