I've encountered a manager like the bullying manager before, but I interpreted it differently.

He was super-super nice to me but would yell and scream at others until the day it was his turn to yell and scream at me.

I read a few articles on Narcissistic Personality Disorder and figured that this was the most likely explanation. I re-framed his early kindness to me as manipulation and his cruelty as his mask being dropped.

This was a him problem, not a me problem.

We both left the company, voluntarily, and I use him as a cautionary tale.

You said yourself that both companies simply wanted to win a client and you were just caught in the middle. You learnt who to avoid (and even took steps to improve your lifestyle), but none of this bullshit was ever about you.