SEEKING ADVICE: My golden retriever killed her sister

Hey everyone,

I have two pet golden retrievers (technically English cream) and I need the advice of an experienced breeder or trainer.

One golden is about five years old, a year ago we had a pet cat die and we got another puppy golden so our first one could have a friend. They got along great and would sometimes play fight but never anything serious, although the five year old has had a record of snapping in the past if you try to take her bone or something like that.

This morning a crazy freak accident happened. They were both playing outside and my mom heard the 1y old yelping, when she came outside the older one was clamped down on the neck of the 1y old and trashing like a pittbull attack kind of thing.

She tried to pull her off but she wouldn’t let go and eventually killed my puppy.

I always loved the puppy more as the older one was never friendly, but what should we do about the older one now? Does it have a mental problem? Need advice please!

Edit; why am I being downvoted I’m genuinely just seeking advice my dog just died, have some consideration please.