[austinjp]

Having been promoted to managing a team I was previously a part of, I feel your pain.

If it helps, consider that this person might have tried exactly the same ploy with whoever was in your shoes. I'm sure they had some issues going on in their life, and issues with the existing management that you inherited unknowingly.

Management stress is real. Part of it comes with maintaining confidentiality, whereas a disgruntled employee will rant and get things off their chest.

I won't say it's 100% about the role and not the person, since that's dumb - interpersonal relationships are real and significant. Equally, it's not 100% about the individual. FWIW, and taking your description at face value, I think your response was appropriate and theirs was not.