My dog killed a dog

Question
My husband and I had three dogs, two females and one male.  They live inside with a doggy door that accesses about a half an acre, which is fenced in.  The three dogs have lived together for six years.  

Two weeks ago I came home to a terribly gory scene.  Our younger female dog (6yrs) killed the older female (9yrs) dog.  We are positive that it was the dog because she was covered in blood and feces.  The male was completely clean.  The dog that was killed was an extremely submissive dog while the dog that killed was the most dominant in our pack.  

She has always been the most well behaved dog and is completely submissive to humans.  She has shown aggression in the past to other dogs, but never to this magnitude.  

Given the small amount of information provided do you have any suggestions as to why this would have happened?  Do you think she will kill the male?  If you were faced with this problem would you euthanize the dog?

She killed a member of our family and a member of her pack.  We are struggling with how to deal with such a terrible thing.  Any help would be greatly appreciated.  We are facing some very difficult decisions and therefor would like as much input as we can get.  Thank you,
Shelley

Answer
Hi Shelly,

I am so sorry this happened.

Female-female aggression is in my opinion far more deadly than two males posturing. While bigger and capable of more physical damage, males will spar for status. Female dogs rarely "bluff". If they fight, they mean it. Their status stays the same through out their lives unlike males who lose their position as they age or are injured.

Given the small amount of information, I could not speculate. It would be grossly irresponsible on my part.

If you opt to keep her, I would never leave them alone together again.