Well, he didn't actually threaten to shoot the student, but was connecting the respect he had to show in Pakistan, which involved the use of a gun, and respect in the classroom. Both were incidents of cross cultural contact.
I could see a reprimand, or even a leave without pay for a period of time. But firing is a bit extreme, IMO. I mean,this is a college level class, and students should be expected to act like adults. Of course, there may have been other ways to handle the student, but I know from teaching college classes, that you resond to these things in the moment.