I think he felt pain when he passed and there was nothing we could do but be helpless and in terror while he was crying out for help or in pain. It was a truly horrible experience....
I'm so very sorry about your kitty. I know what you went through. I lost my one cat this spring and she died at home. It was on a Saturday and the vets office was closed.
My vet is one of these that feels that in some cases it is perfectly o.k. to allow an animal to pass on its own. I'm not sure that I agree with that after experiencing it. He did tell me if she died over the weekend that she would only feel pain in the last 20-30 minutes. I could tell that was pretty much correct as she was completely unresponsive but then about 20 minutes before she died she came around and was restless. She got up and was moving around the room and would cry. It was very rough to watch.
I also lost my dear Dobie girl in January. The stupid vet that we were going to then told us she was fine and didn't need to be euthanized. A week before New Years DH called him and told him she wasn't doing well and he thought we needed to bring her in, so he did. Again the vet said she was fine, no worries, etc. The evening of January 6th she passed away. It was rough because she hadn't been herself all day. She didn't seem to be in pain but in the last few minutes she panicked. She knew something was happening and she kept looking at me with this desperate panicked look in her eyes. I'll never forget that. :'( She passed away moments later.
I have also taken pets in to be euthanized and have not had that go well either. My old sheepdog panicked and fought like crazy. I struggled with that for along time as well but there were no options in her case.
I don't think there is ever the perfect way to let them go. Its always going to hurt and sometimes there are a few bad experiences along the way.
Try not to feel too bad over the way things happened.