First, take comfort. You are not alone, especially these days, in having a chapter of your career that you would like to leave out of the story. All kinds of people get into difficult work situations. But few know how to talk about them in a constructive manner. So look at your personal nightmare as an opportunity to show your communications skills and your professional judgment.
Your answer should be short and honest. You don't want to sugar-coat the truth. But you do want to show that your motives were positive and your actions were based on a clear assessment of the situation. Highlight what you learned in the process and how it will shape how you approach your next job. Self-awareness is a valuable trait in all aspects of your life, including in the workplace.
There is a tendency in these situations to over-explain. Resist it. Stay away from personalities, from gossip and from the need to place blame. These are traits employers avoid like the plague.
Finally, think your answer through in advance and get comfortable with it. If you handle this part of the interview well, odds are the interviewer will respect you more not less.
Jean Allen, partner at Exchange Place Partners.