Thanks for the tip, Greg. I did post a reply to Don, and if I do say so myself, I curbed my temper quite nicely.
This chap, Don, has a bad case of Borderline Personality Disorder. It's characterized by the belief that the rest of the people in the world aren't real people--they are just extensions of oneself, like one's Teddy bear. Therefore, other people are expected to experience things in exactly the same way as oneself would. And it is perfectly justifiable to shout at others and call them names when they do something unexpected.
I wanted to tell him that it wasn't his place to tell others what they are or what they should do, but I know from experience in dealing with people who have BPD that direct confrontation only escalates the conflict. *sigh*