I'm going to share private information an ex-friend shared with me
Do you have a good reason to break their trust?
I want to cause trouble for him.
If this person is indeed your friend, you should not share their secrets. Doing so will only cause them to end the friendship.
Teach him a lesson. He shouldn't trust anyone.
If other people see how you act when things go south they may find you untrustworthy. Even if it hurts you should keep their secret safe as an integrity issue.