Sudden death at the vet due to negligence

I took my cat to the vet because she was breathing heavily and lost 2lbs. She was behaving normal otherwise, good appetite, still playing, normal bathroom activity, etc. The vet took a chest X-Ray and said she had some fluid around her lungs and it could be heart disease or cancer. The vet sedated her to drain the fluid for testing. She sedated the cat and left to see another patient. I was sitting with my cat who started acting weird and I got the vet tech. The vet came back in and said it was nausea and injected her with some anti-nausea medication. Shortly after my cat started being really wobbly and foaming at the mouth. The vet said it’s normal. I was feeling really worried but I didn’t know what to do. Suddenly the vet grabbed the cat and said she needed to be intubated. The cat fought it for a few seconds and I could see her tongue was blue. The vet didn’t have a tube small enough for a cat so she never was intubated and she died. I watched the whole thing and it was the worst thing I’ve ever seen in my life. I can’t stop playing it over and over in my head and I blame myself because my cat was so scared and I was telling her it was going to be ok and she trusted me and let the vet do all these things to her and ended up killing her. I realize she was probably sick but she was nowhere near the end of life. She was only 6 and she was playing on my patio that morning. She wanted to live. I don’t think I can ever forgive myself and I don’t know what to do. Does anyone have any advice? I’m thinking of taking her to get a necropsy to try to get some closure. I have her body in my deep freeze; I know that’s weird but I couldn’t leave her body at that horrible place that murdered her.