You did the right thing separating your horses
There are some horses in this life that are just bullies.
My horse #3 of 4 is a bully - always has been but he's gotten better now that he's older (he's 20). He wants to be the leader and is hesitantly being groomed for that position by the strong alpha who is a long 26.
Bullies are not being kind to other horses because they want to be, they are behaving because those other horses will knock the snot out of the bully if it doesn't mind it's manners.
My 13.3H 28 year old Arab is #2 and what is known as a "passive leader" but he does not have a bit problem taking a hunk of flesh out of the 16.1H bully if he has to - lol lol
The bully horse used to try and run my #4 horse thru the paddock corner if he got up on the wrong side of the sawdust that day but he's pretty much stopped that nonsense.
Just as an FYI with your horse, that might be of some help:
My bully horse is grain and soy sensitive so, I removed all of that from his diet. He is on a grain and soy-free ration balancer which is also low in protein and fat. I don't worry about low protein and fat in the feed pan because he is on 20+ acres of pasture:)
He also appears to be magnesium deficient as he has really stopped going after the #4 horse since I started him on Remission. Remission is magnesium and a lot of Vitamin B that is fed to horses with suspected insulin issues.
I started feeding the Remission as this horse shows subtle signs of insulin problems, I had no idea it was going to make such a kitten out of him, which he has even stopped going after the cats when they slip thru his stall
I'm not saying these are issues with your horse but while he's separated from the rescue, it wouldn't hurt to "rat experiment" with him on the feed and the Remission. Remission won't hurt him, he will dump it from his system if he doesn't need it; you will just be out $20 or so for a small tub at Tractor Supply