NTA. Him doing that behind your back is creepy and I definitely get being upset and angry! That is very valid! However, you mentioned he cut off Leon and said Leon is awful, right? It seems likely that your bf was in a toxic relationship/friendship with Leon, given how he both acted to try and impress Leon by making stuff up, but also to protect others from him. It's up to you and definitely out of this sub as to how you continue, bc that's definitely relationship advice territory, but talk to your bf and explain what you feel, and let him talk about this whole thing with Leon. I'm sorry this happened and again, NTA but it is a very yikes situation & hopefully given your bf seems to be out of contact with Leon and not doing stuff like this now, that may help.

It sounds so disturbing that they were just making stuff up to each other. NTA!