Hey man, for what it's worth, I can empathize with your situation, and I know it's really hard to do this.
I blew up a close knit friend group because 2 different guys in it turned out to be extremely shitty. One did something very similar to what your ex friend did and the other was a controlling abuser.
After the dust settled, the neutrals in the group kinda reformed it without me, and disgustingly, from what I know they kept the 2 assholes as friends as well. They all choose to look passed what happened to this day.
It's going to hurt for years. It's going to be lonely, but you did the right thing and you have to remember it. You will be confronted socially with being an adversary and that makes you feel wrong and like an asshole, but you aren't.
I still have a hard time with it. It was my core friend group of what I thought was ride or dies and I'm not in it anymore, because I had had enough and spoke up. I got punished for it.
There will be whirlwinds of emotions and is okay to feel all of them.
I'm sorry this happened bro
Mourning is absolutely the right word for it. It's the death of a relationship and of an era for you. Just because the individual isn't actually deceased isn't going to make it hurt any less.
For me I definitely felt a lot of that, along with anger and betrayal, sadness... Frustration.
If you want to vent about it feel free to dm me.
I hate that I don't have friends like that anymore, even though I know I did the right thing. I sucks so fucking much