[HELP] Our golden retriever killed our other golden retriever

Hey everyone,

I am honestly so heartbroken writing this post, I live with my mom and she has one golden retriever about five years old who was always fairly normal, occasionally demonstrated some resource guarding like one or two incidents where she would bark if you try to take her bone, that's about it.

A year ago we picked out a second golden from a different breeder and she was the sweetest girl I ever knew. She was so innocent and loving and cute, I loved her so much and she was my best friend as I never really cared for the other dog because it wasn't nearly as affectionate, and the puppy I picked out myself. She was the runt of the litter and smaller than our other dog.

Yesterday the most horrible thing happened, I woke up to the sound of my mom shrieking and I looked out my window and saw a neighbor rushing my little dogs limp body to the car, I called my mom and she told me that they were playing in our fenced off yard when she heard barking, then she heard cries for help, and came out to find older golden had latched onto her neck and she tried to pry her off but she kept thrashing and by the time she let go my little one was convulsing and probably couldn't breathe (she died right there).

I'm so heartbroken and I miss her so much, I know we need to get the older thing checked out by a vet and behavioral specialist, but does anyone have any idea as to why this happened, there were some builders in the yard laying bricks but they didnt have any power tools and weren't near the dogs.

I miss my dog so much, she wasn't even a year old and it breaks my heart that she had to suffer.

I just want her back