Yepp. You have to. Our dog was at an Emergency Vet a few months ago and they "Did everything they could". We had to pay over $900 for a dead dog.
Let me see if I understand your situation.
Your cat was in a fight and you took her to a vet that said they opened at 6AM and you waited until 6:15 AM and drove off. (When did your cat get injured, that morning? last night, days ago?) Then they called you at 6:30 AM and said they were open.
So you went back and dropped off your cat? (you didn't wait to talk with the vet or be seen with the cat?)
They didn't call you, but you asked to be called if the cat got worse.
They didn't even look at the cat for at least 4 hours after you dropped her off????
They didn't call you to tell you they were keeping her overnight???? They just wanted to "watch" her???
She didn't eat or drink for 24 hours???? At home or the vet???
Then she died.
I think there is a lack of communication between you and your vet.
If the vet kept her overnight, they must have had some reason/diagnosis. No, they don't watch them every minute for 24 hours. She must have had IVs and medications.
Vitamins don't treat injuries from a fight.
Call your vet and get more information and a better understanding of why you have a $800 bill. By leaving your cat, you have given consent for treatment and authorized a bill.
I understand your anger and I am sorry for your loss.
But you did authorize treatment.