It’s a deeply strange, cowardly and cruel thing to do.
Totally understandable that you feel completely blindsided, shocked, heartbroken, angry and any other emotion that’s coming up!
I think in these situations - where someone behaves in such an unexpectedly shitty way - you could drive yourself mad with wondering why it happened. Were there signs you missed? Is there a big part of the picture you don’t have? Does he have mental health issues / a secret wife / a terrible illness? Is he a serial future-faking shitbag? Blah blah blah.
Honestly, though, I don’t think you will ever get the answers you want and (and I know it doesn’t feel like it now)…they don’t matter.
You behaved like a decent, open-hearted person. He has truly shown you who he really is, which is quite the opposite.
I would be tempted to send him one brief, final message along the lines of: Having reflected on your message further, please return my stuff to X by X date and do not ever contact me again.
Then block. Delete. Scrub every trace of him from your life. And then allow yourself a fixed period (a weekend? A week?) to cry, rage and tell your friends, then make the conscious decision to erase him. And repeat this mantra: It was HIM, not me.
Some people are just snakes. It is not a reflection on you. It really isn’t.