How should I treat an "ex" who was verbally and physically abusive when he's at a party I'm attending?

This really bothers me. I was married for twenty years to a man I was convinced was “perfect” in every way imaginable. After fifteen years he not only changed into a violent, frightening, abusive jerk it also took that long for me to see what he had become. Maybe because I was volunteering at a women’s crisis center. Maybe because I had grown up. I never, ever fought back. I just ran and hid. One of our daughters was psychologically damaged from all the insults, profanity (on his part) and crying. Now, twenty nine years after the divorce I so wisely attained, I’ll see him at a party on rare occasions and he always is so friendly and smiling, showing me pictures of our daughters in his camera and cracking jokes. I’m friendly also but afterwards I ask myself if I should have been nice and accommodating towards him to reduce or eliminate tension at a family party. How would you act towards him? I suppose I’m neurotic or nuts still ruminating over his Satanic behavior but it really devastated me and I now have an anxiety disorder which began the last year of our marriage. He remarried a few years ago.