I think a decent company should treat a decent employee with respect and professionalism in all situations, and vice versa. Someone being fired is no different. If you drop all trust in the professionalism of an employee who has not given you reason to do so, it's pretty much the same as saying you never trusted them to begin with. Ugly, like an employee who can't leave properly.
I have been fired once. I was competent in general but ended up in a position that was a terrible fit for me, and the work product didn't turn out well. My boss saw the situation for what it was, appreciated that I was putting in a lot of effort to turn things around, and tried to help. We didn't find a solution fast enough and at the end he had no choice but to let me go. When he broke the news, I suggested he'd date it to the end of the week because it would take until then to wrap things up properly. So he did. On the last day, I went around to exchange goodbyes with my team. Then worked for a long time until I felt my work items in progress were squared away and all of the little useful data from my head was documented for whomever would succeed me. Sent a final status report before logging out. Packed up my stuff. While heading out I visited the IT department to return some computer things, and made a final stop at the security desk to leave my keys. I also left post-it notes with these items, because it was late night and I had been the only person in the building for hours. Until the moment I walked out the door that night and ceased being an employee, I had access to all of the source code and other IP of this software company plus physical access to the building with ~100 high end workstations. The company didn't feel a need to insult my professionalism by withdrawing their trust, nor did they treat me as anything but a valuable employee at any point. For my part, I kept acting like one and giving them my best effort.