What is the proper etiquette if a person unexpectedly meets someone he fired? For example, if a person is at Starbucks and meets a former employee that he fired what should he do?
Sometimes situation may dictate not saying anything but in most cases a simple "Hi how have you been?" is all that is required. It is impossible to know what is going on in that person mind and at that point just be normal like anyone else would.
Now if the firing was for something egregious or deplorable then staying silent is probably best.
That said if they engage you don't be rude and ignore them. If they say hi then say hi back and just be polite and move on.
I would like to end with this. People make mistakes that end their career in a company but that does not make them bad or any less human. People often regret their mistakes and just want to move on.
Depends on the person and the circumstances. Obviously if they're drunk or drugged that could create an issue.
But in the normal run of affairs, just ignore them and let them initiate any greeting. You'll be better able to judge the correct reaction based on that. Obviously a bunch of swear words while they roll up their sleeves is probably a bad sign.
Some industries would have different things to watch out for. If you sack a bouncer for example, there is a chance of a violent reaction at some point.