I completed my masters in mathematics from Asia in August 2021 and joined PhD in Germany in Nov 2021. I had to do a masters project/thesis in my 4th semester (April-August) and for that I asked a prof. in my country but a different university in Jan 2021 as no one in my university was specialized in Algebraic Topology and he (say Mr. X) agreed to supervise my master's thesis. One prof. (say Mr. Y) in my university whose research interests align remotely with mine gave me a LOR to submit to X which he asked.
I visited X's university in March for 4 days. He met me on the 2nd day for about 20 minutes. He asked me 3 questions and I couldn't answer one of them because of difference in definitions (the term was not available in the book from which I studied). Next day he e-mailed me the link of a paper and said he was busy and didn't met me.
I studied the research paper and mailed him 2 questions which I had on May 15, 2021 and asking when should I visit him. Getting no reply, I e-mailed him again on May 30 and began reading two papers related to the 1st paper in spare time. I e-mailed him again on June 15 and getting no reply I realized he will not reply to my e-mail.
I was morose and for a week didn't attend any classes and only ate 1 time a day and didn't go out of my room, was really angry and depressed. I burnt my 2 books by putting them on fire in the room. Then, I called Prof. Y explaining him the situation, and he said I should remove X's name from thesis and I should add his name as some prof's name must be on thesis due to dept. rules. I had no guidance for my master's project. I wrote an expository style thesis explaining what I understood and got 2nd highest marks in thesis out of 32 students.
I also e-mailed him once again in Oct 2021 to review my thesis but he didn't reply.
This guy is one of the senior profs and is at the top university in my country. Prof. Y is much more junior to him and so are the other prof.'s (say P and Q) who wrote LORs for me. I didn't complain about him in his university or my university as the culture in my country is fascist and it is ingrained in culture to suck up and respect elders and seniors. Also, I am from a persecuted group and the ethnic group of prof. X (and Y, P ,Q) has absolute dominance in academia and society and are persecutors. People here belief in a lot of irrational stuff. It was unlikely that anyone would have supported me. This prof. X has a lot of contact internationally.
This incident had a very deep impact on me. Also, I had to lose face among all 3 prof's who gave me LORs and my classmates. But I couldn't do anything.
I sometimes felt like verbally abusing him over a phone call or writing a mail filled with verbal abuses. But, this guy would have definitely called prof Y and my dept and that would have definitely badly impacted my career.
I think this guy has no conscience and shame and is absolutely filth of a human.
Now this prof. is coming to my university in Germany to give a talk in my dept. and my advisor is the only prof whose interests align with him.
When I meet him, I am sure, he would say that I was not good enough when he asked me questions at his university in March 2021 or that I lied to him about my qualification. (I had done 2 internships and 2 courses in topology before asking him for supervision).
The only thing that comes to my mind as answer of this question is "HECK yourself, you piece of filth" . I think I will be really angry in that scanerio. But, other people there will not be pleased and he will definitely tell Prof Y, P, Q who gave me a LOR.
When I imagine him in elevator alone with me, I think I might punch him or slap him, if he starts talking about how or why he didn't supervised my thesis. I will get involved with the law.
How should I engage with this person when I will meet him which doesn't harms my career?