Unfortunately there's not always an answer to "why" i don't think BD. I've been sober for about 6 and a half years now and things still happen to me that I don't expect. Granted I've made a ton of progress in all facets of my life - mental and physically but I still have work to do.
Regarding the specific situation, sounds to me like it's been building up for a while. Bottom line, you let this guy rent space in your head and you should have evicted him much sooner in the process. All those things you list about him - the patholocial lying, etc - it's utterly irrelevant unless you allow it to be. You can't control it and you'll never be able to change him, so it's best to not even engage him in the first place.
If you recall this whole situation is somewhat reminiscent of your inability to let go of the situation with your father a few months back. You wanted a certain outcome for his life that was not the same as what he wanted it caused you a overwhelming amount of angst. Sure you didn't physically mix it up with him but the situation was pretty much exactly the same. In both cases you did not agree with what was happening and wanted to change someone else - which is impossible.
I'm glad you were able to leave before things got too out of hand. Moving forward though, for your own sake, you've got to figure out a way to just let go of wanting to control other people. It will drive you to your wits end.