Professionalism cuts both ways. Any company employee that reacts negatively in ways similar to what is described here (when the interviewee announced he was leaving) would display what I consider to be extremely unprofessional behaviour. The fact that this reaction seemed to have been shared by a number of employees is perhaps a symptom of groupthink. The best feedback ever I got when we were in the process of hiring was from one of my very laid back employee, who is often dismissed as "unimportant" and has a way to put people at ease, and who got some unique insight into what a prospective employee would be like.

I see this type of interview a typical "alpha male hehaviour competition" where existing employees are trying to assert in advance their worth to a potential future colleague (to put it nicely).