Bullied to death

I have had the same flock for 10years now and slowly they have died off and I was down to my last 2 hens. One of them has always been aggressive but nothing over the top all of these years-never drawn blood. Well she has been broody lately and a bit of a pill and I guess she just decided that that particular day she was going nuclear on her sister. She peck her vent so bad that the insides were out. It was traumatizing for the poor sweet bird and for me. I had her euthanized. Now I have one lone bird who is still being a witch and I am so traumatized I can barely look at her.
I’m afraid to find her a friend because she is a bully and I can’t believe she went as far as she did. I don’t want to out another bird through that. But I also know that the lone bird won’t do well without a pal especially since winter is coming and it is boring and they don’t free range as much.
Has anyone had any experience with this? I need some advice please.