Nutshell: My ex and I are coparents of a teen. My ex has partners coming and going all the time which frustrates me because they are 1) demonstrating poor relationship behavior in front of our kid, and 2) our kid gets attached to all of them and then comes to me for comfort and questions when they inevitably leave. 

A couple days ago my ex and I got into a minor disagreement while dropping off the kid. My ex had their current "friend" in the car with them and the person rolled their eyes at me.  I chuckled and casually and sarcastically mentioned that they might want to read up on being in a relationship with a narcissist. I walked off before I got the full reaction from either of them.

Now my kid is saying that person isn't coming around anymore and I'm torn between smugness and a little guilt. But I would have appreciated if someone had warned me from the beginning about my ex's illness (because obviously they weren't ever going to) so that I could have informed myself and been better prepared for that relationship or left earlier. So... AITA?

(I'll add that ex really does have NPD, I've been through a massive amount of therapy to overcome some of the things that happened in our relationship. I'm not just trying to sabotage their relationships for spite).