Whenever you use words like "bitter," "mean," and "insulting" to describe an applicant's comments, that typically indicates that this is not your ideal candidate. While you could possibly give him high marks for openness and candor, he does not make the grade when it comes to common sense, self-control, and empathy.
You should also take note of the fact that he directed his venomous comments not only toward one previous employer, but also toward most of them. This is usually an indicator of a person who does not take responsibility for his own actions and is quick to blame others for his failures. If you hire this individual and he then leaves, you can also figure that he will be extremely negative in describing you and your company, too.
A job interview is a time for an applicant to demonstrate his or her skills, knowledge, abilities, and other strengths, and to do so in a positive, confident, and optimistic framework. If your applicant was aware of this fact and still poured out such negativity, that's a red flag. And, if your applicant was unaware of this fact, the red flag is even bigger.
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