Returning dog to shelter

So we adopted two dogs from our local shelter last month Both are 2 years old and were kennel buddies in the shelter, they are lovable and sweet. Their paperwork said they were both okay with cats and good with other dogs, well on Friday, one of the dogs killed the family cat when she broke her collar and the other dog tried to help her. Then today, Sunday, attacked another family dog, almost ripping his ear off while the other new dog tried to attack him from behind. The dog who starts the fights refuses to let go, no matter what. I ended up breaking my hand trying to stop the fight. My question is, we've talked to behaviorist, they think it would be best to either return to shelter and hope for a family with no other pets to adopt them or to euthanize them. I'm so torn on what to do. I would love for them to find great homes that can provide a single pet home for them, but I also don't want another animal to be hurt. Tell me your opinions please. Please no bashing, I've reached out to a few trainers who have said they will not take these two on because if their tendency to not let go. Which would be best, return to shelter and pray they find a good home or to euthanize and make sure there can never be another incident?