Attending a conference that my abusive ex-supervisor is also attending, how I should react? I am going to attend a conference as I have been selected to a voluntary position and it is mandatory to meet up with the organizers. At this conference, my abusive ex-supervisor is chairing sessions and probably he is going to attend, he does not know I will be there.

I hate him so much as I was forced to leave because he was racist and narcissistic, travelling to another country with harsh conditions, it is a long story. Now I am working on the same topic with another nice supervisor.

No one knows what happened, but I am afraid of this encounter, I am so emotional and I hate him although he apologized via email after I left couple months, but I still have a grudge since he delayed my academic progress and I cannot tolerate with that, how I should react, ignore him completely, he is very awful person.
The best revenge is living well. You have a good start on that working on your topic with a nice supervisor, and attending the conference as a volunteer.

Ideally, treat him as just another academic you happen to know, about on the level of a professor who taught one of your undergraduate courses. I understand that may be difficult, but it tells him that his treatment of you was just a minor roadbump in your life. He is too unimportant to be even worth your anger. For a racist, seeing you happily getting on with your life will hurt far more than if you let him see he still has emotional power over you.