In an interview is it okay to ask the interviewer why the previous employee left the position or was fired from the position I am applying for? I would like to understand why someone would have quit a job or been fired to better understand if the position is right for me. Many companies these days do exit interviews so they probably got an answer from the last employee before they left. Is this considered rude? Am I likely to get a meaningful answer or simply a generic answer to avoid the question?
I have both asked and been asked that type of question without apparent harm. (I've gotten offers after asking.) However, your phrasing is problematic; the better way to ask the question is "why is this position vacant?", which covers two cases you haven't considered in your question: (a) it's a new position, and (b) the previous person was promoted. Don't assume that something bad happened; it makes you look like you're jumping to the worst interpretation, which might give an interviewer pause.
If you learn that the previous person did in fact quit or get fired, I recommend against asking why. You don't want to make the interview about that other guy but about you. (You're also unlikely to get a useful answer; they don't want to talk about that other guy either.) But be more alert for clues about culture, unreasonable expectations, etc, and use your questions to dig into whatever aspects of the job you're most concerned about.
There is nothing wrong with asking an interviewer why the previous employee left or was fired. In some cases, you may get the actual story. However, you should treat the answer the same way you would treat the answer you get from someone you're dating when you ask them why their previous relationship ended. You're only going to get their side of the story. If they were the reason their previous relationship ended, you'll likely not get the complete truth. "We discovered it wasn't a good fit." "They decided to seek other opportunities." "They had a change in their personal life which necessitated a shift in their career direction." Sometimes they will slip up and reveal something unintended, which is why the question should be asked, but don't expect the full story.
Before ask this question, I would ask my self what the person would answer to me. In that way, you may able to phrase the question in a different way according to that person. Do not make any inquiry without knowing possible answers in the interview place. Don't expect them to say to you that - 'We gave him hard time, so he quit, now you are next !!'