Put yourself in the interviewer's perspective for a moment...
As soon as you badmouth your former employer, or any employees of your former workplace, the interviewer will start to wonder what your employer would say. They’ll always be thinking to themselves they want the other side of the story.
It won't matter how "honest" or "forthcoming" you might think you are being either. There's always going to be a certain amount of doubt remaining about your story in the interviewers mind anyway you choose.
One of the problems is whenever you provide them with a detailed explanation of why you left your former employer, that story becomes the primary focus of the interview.
That is to say, the interview is no longer about "you" in the scope of being a qualified employee for the posted position anymore. It becomes about "the story" why you left your former employer instead.
Additionally, everytime you answer one question about the story, it just creates more unanswered questions for them. There's just no end to it and it sabotages the interview.
If the interview continues in this direction, the only possible outcome at this point is for the interviewer to ask themselves questions like:
#1 “How do I know this isn’t going to happen again if I hire this person for my team?”
#2 "I wonder why this employee didn't get along well with others?"
Then the interviewer at this point will probably just want to "play it safe" and think it's better not to take a chance, rather than make a hiring mistake. A mistake that would make them look bad to their boss. After all, they have many other applicants ready to interview and it's just easier for them to look at another one, instead of trying to confirm or deny any details of your horror story at your former workplace.
That's why it won't matter what explanation you provide them with. It just leaves them with "negative thoughts" the more you talk about it during the interview.
Why poison the interview? The interview should remain positive, progressive and be moving forward at all times. If it stalls on "the story" then you most likely won't get the job.
Always better to divert the interview away from that discussion and get off the subject in the shortest amount of time possible. You can do that by providing a short, professional and reasonable response.