Posting in this bowl because I feel like the comments will be nicer and more helpful. I am about to give notice to my managing partner. I am lateraling to a competitor firm. My managing partner and I have a good relationship but I have seen how he has responded when others have left and it was not good, to say the least. Looking for talking points to make my resignation conversation as short, professional and sweet as possible (and any words of encouragement would be nice too)!

I would put the resignation in writing because he seems like a psycho.

In person: Thank you for the past x years at the firm, I’ve decided to accept a new opportunity and will send an email follow up for official written notice. My last day will be x.

Then you send the follow-up letter. There are templates online. There’s nothing wrong with taking a new job, if your employer can’t handle it then that’s a sign moving on is a good idea.

Don't tell him where you're going. They ask, but you don't need to tell them.

Is it possible you can just go through HR instead of speaking directly to him?

Remember you can say as much or as little as you want. If a niche practice area you are likely to encounter the partner (and firm) again so doing in a way that leaves both of you with warm feelings is your best bet. Be gracious and polite and you can just say you wanted exposure to a slightly different practice or people to continue growing.

In person then follow up in writing. I thought people were going to be mad at me when I resigned, and most were genuinely happy for me. I was changing the focus of my practice a bit, which I could not do there. More contracting, less litigation.

Never burn bridges- you should know that by now

Imagine yourself as a different avatar: Buy him a bottle of good wine or whisky,buy a card, write some funny memory in it,say: let's stay in touch !
Go to his office with a friendly smile on your face and say good bye words as a person to person as you are not in a war zone with Afghanistan, after all you worked with that person.
The goal here for two of you is to pass an exam to act like you were born to be: act like a human and be polite reflecting your true inner self of how you would like to be treated in a similar situation if you would be him
Once you gonna accomplish that : say to yourself : I'm ready for a new adventure ! Thank you Universe/ God for this experience

As for him,his story is deeper : as long as he lives he will face a rejection,abandonent and cognitive dissonance which is : I'm hurting but I must appear okey in the eyes of others
Well learned behavior from a childhood when a parent left and he had to pretend that everything is ok in the eyes of others ,he needs to talk bad about that person to reassure his avatar that they have been there before,they survived and they will be ready for another hit in the future
He needs a psychologist to work on his trauma but he will never admit that there is any because that imprint got so deep into his avatar that is hard for him to recognize who he is and who is he not ?
Well, good luck with your new adventure!