I'm sad that you had to put the hen down- but hopefully you can take some comfort in the knowledge that you were trying to do what was best for her.
As for the fisher (they are giant weasels)- they do not normally prey on large animals like chickens. It is odd that you saw it at all, let alone...
I fear she is in a great deal of pain. It might be best to put her down for that reason alone- no need to let her suffer.
It is a very hard situation. I'm sorry to hear she was attacked.
-MTchick