BLUF - How do you respond to the question about being terminated?
I worked for local government which can be very political. I was fired for 'gross insubordination' which is not in my character at all.
Background:
My boss was an alcoholic and abusive. I went to upper management that did nothing. One day we had a local emergency and a disaster was declared. My boss told me to come to work at 10 PM to man our emergency response.
However, during an emergency I am to report to the command center (911). There were three of us that were trained to handle emergency operations and one was on vacation.
Short story - I refused on grounds that I had to handle the emergency operations. I was disciplined and subsequently fired. I fought it but the politics were all over the hearing.
A year later my old boss was fired because of his management style.

Not that way.
To be blunt: if an interviewer senses ANY SHRED of the bitterness or excuses that you've put in your post, they will NEVER hire you. Every point you made on your post (without exception), reflects negatively on YOU, not your boss or your company.
Mark said it best in the "Getting Fired" cast, "There are smart things to do, and dumb things to do. To me, this is a test of your emotional maturity"
Listen to that cast. But more importantly, think about your own behaviors and attitudes that led to this point. Use this as an opportunity to improve yourself.
Then let it go.
Hi KS, I think I would
Hi KS,
I think I would say that part of my job responsibilities required me to respond to such-and-such command center in the event of certain emergencies or disasters. At the time I was responding to the command center my supervisor called me and ordered me to come to work to do ______. I went to the command center as per ___(whatever rule, requirement, previous order, etc.)____ instead of reporting to my direct supervisor. As a result I was terminated.
I would not make any justification or excuse or reasoning behind your choice at all. If you are asked for facts give facts. I would expect the interviewer to follow up and ask why you chose to appear at the command center instead of report to your boss and that would give you some limited opportunity to explain your actions. For example, this procedure was put into place by people with much more knowledge and experience than I have. It was stressed to me and everyone else that it is incumbent on me to report to the command center to carry out tasks x, y, and z and I knew people were depending on me to report.
If pressed I might say I reported to the command center as opposed to my supervisor because this procedure was put into place not only by people with much more knowledge and experience than I but presumably also in consultation with various other emergency services and in a calm state of mind as opposed to the stress and uncertainty of an emergency. Maybe I should have reported to my supervisor but I chose to report to the command center because I was previously ordered to do so in this type of emergency. I was stuck between two conflicting orders and I chose wrong. Of course, had I obeyed my supervisor and ignored my responsibility to report to the command center I might have been terminated as well.
On a closing note, I would think that an interviewer can sympathize with contradictory orders during an emergency. However, I totally agree with Mike that your post reeks of bitterness and excuse. If you mention that you didn't report to your boss because he's an abusive alcoholic then I would assume you disobeyed his orders because you thought you knew better, not because of a prior order. Your former boss my be incompetent and he may be a jerk but complaining about him isn't going to help you get a job.