I had the problem of a dog fight between two Jack Russell terriers. They are noisy, scrappy, dogs and some snarling, postering and scuffles are par for the course. When you combine 3 households of terriers, we used to say it wasn’t Christmas without a dog fight. But there is a difference between the loud scuffles and the more deadly and often quieter fight with death in mind.
We had two lovely females, who got along with each other, cats and kids. Until one day the older female decided the younger one had to go, she jumped her and was seriously trying to kill her. Once we were able to separate them, treat the younger one, they were separated until we were able to rehome the aggressor. With small children at home, we couldn’t take the chance of serious fights, and we knew we were lucky to have been home to break up that fight.
We placed the older dog with a retired couple with no other dogs. They were fully aware of the fight that led up to her new home. She was much happier in her new home, without small children and annoying younger dogs; with humans who had time for and doted on her. We set it up as a trial and would take her back if either party was unhappy. Apparently, Thistle looked around as if to say, “finally peace and quiet!” We received Christmas cards from her until she crossed the rainbow bridge. My last terrier came to us because she was beating up the bull dogs of her household. This time we were the quiet single dog household, and Tuppence settled in without a backward glance.
OP, I hope you can resolve your situation in the best way possible, whatever that may be. Jingles for you and your house.