I'm going to inform people why someone walked out on us.
Why would you tell people someone's private business?
Because I am pissed that they left us.
Being pissed off doesn't give you the right to tell everyone a person's reasons for leaving your group. Betraying others confidences shows that you are untrustworthy.
I guess you are right and giving them no energy is best.
I think you are making a good choice. Sometimes it is best to ignore people's discretion as that shows grace.