How do I know if a vet euthanized my cat without my consent and just trying to cover it up?

I have reason to believe that my cat, whom I brought to the ER to be more or less stabilized after his chest was tapped of fluid by another vet seemed to not have helped much, maybe got him worse, we transferred to ER to get a little help so we can take him home and he can die at home. However, we think the vet took it upon himself to put him down because when we walked into the treatment room, he was on the table on his side, and 4 techs were around him, 2 seem to be holding him down at least. When I picked him up, he was like jelly when beforehand, just 30 minutes ago, he was still alert. He was weak from the tap, but he was alert, and their own tech told me he was "stable" in the oxygen cage. I'm considering an autopsy, just to get closure, but not sure whether it will even yield anything. All we wanted was to be able to take him home so he could die at home, the way he wanted, by his favorite spot at the front door. He cried yesterday when we took him off his spot to go to doctor