First, I tried speaking to this person. Making sure I am not assuming things. Took him to a conference room and genuinely told him how he is making me feel. Things got better for a week and then back to same old.

Second, I escalated to the partner. Gave supporting evidence. Including twitter thread from someone who had quit and basically mentions everything this person has done. Partner "tried" mediating. I mention "tried" in quotes because it felt more like a you have misunderstood him, the practice is demanding etc etc. My take-away was he would rather lose an SA who has joined the team and has been working for 3 months, than a PA who has been in the team for 1 year and caused 3 attritions within that year (all people directly reporting onto him).

Once second was clear, I knew I would quit. So I did and moved out. I did mention and flag this guy in my exit interview. I am told he never got a promotion later because of that and had to quit and get partnership in another firm. Long story shirt, he is an equity partner today in a T2 firm and people continue to quit under him.

The morale of the story is, control what you can control. If the atmosphere is not sustainable, quit.

Here's what I found - there's nothing you can really do other than make your own personal situation better in the team, so disarm with charm and kill with kindness. Seniors can only be horrid that many times in the face of absolute non-retaliation before they consciously self-correct. - M.K. Gandhi was onto something for sure.