Anyone can become unstable if raised in a bad house. People skills used logically to perform acts of revenge doesn't sound INFJ to me. If what you are saying is true, this person sounds like they have mental health issues, and is a bad egg - people like that are bad news and I cut them cleanly out of my life. People of any type that don't respect boundaries then need to have clear and consistent walls put up between you and them.Why INFJ? Because I believe an INFJ can become mentally unstable if raised in a bad house (as he was). I believe an INFJ has the people skills and logic to perform Shakespearean acts of revenge on the level of Othello.