I understand that these people are human and have their good and their bad days.
However, they are also professional actors doing a job. I don't know about anyone else here, but "I'm having an off day" would not likely be accepted as an excuse for very long in my job. And I'm willing to bet that when any of these actors are on a film set, they are expected to be able to deliver a good performance regardless of what kind of day they're having.
These folks aren't showing up at conventions out of the goodness of their hearts. They are being paid rather substantial fees for their appearance, and the folks in attendance at the convention have paid to be there. How is it any different than if I paid to attend a Broadway show and one of these actors was being paid to perform in it? I doubt the producer of a Broadway show would accept "well, they're human, they're having a bad day" as an excuse.
Sorry, but I'm not willing to waive bad behavior on the part of these actors off as easily as some here. We all have personal lives and we all have problems. But if you're being paid a hefty sum of money to entertain and interact with the fans, then you'd better damn well be a professional and do just that.