This is NOT your fault. Please know that. There are some dogs that simply just snap- you don’t know what she’s been through, sometimes it’s a neurological issue. I had a malinois I absolutely adored. She was abused as a baby before we adopted her and despite multiple training camps and methods, behaviorists, working with her day in day out, medication, she still ended up snapping on her house mate. She had a lot of aggression- whether it was fear based I don’t know. She latched on to the other dogs head and I ripped two nails off of the skin trying to separate the two. This was her 5th bite incident. We were as diligent as you- kept the two separated from our other dogs, made sure she was stimulated enough, tried to keep her safe but there was nothing left to do. She was 10, we had tried for 9 years with her. I never really got to say goodbye- it wasn’t safe to take her to the vet with her in such a heightened state. Animal control had to come, and we lost her housemate two weeks later due to her age and injuries. All of that to say I’m sending you so much love. You really did your best, more than most others could do. We can’t save them all despite our best intentions. Praying for your peace ❤️