We have over 10 cats that passed away in our care.
One died of a heart attack last year and my son told me she was in pain for about 30 seconds then passed away.
Two died in the pet hospital - one by heart attack and the other has fetal diabetes and had to be put down.
One suffer a stroke and forget who she was. She lived for another 6 months and passed away peacefully on my wife's lap.
There were three of them that died on my lap. They were very weak but still tried to look at me. ( apparently asking for permission to sleep). I padded them and gave signal it was ok to sleep. they are died peacefully.
Two of them would hide inside bathtub (cooler) for days. (They had fever and the bathtub gave them some relief from high body temperatures. The one that passed away in March this year had bone cancer on her chin. She stayed in the bathtub for the last two week of her life. .She passed away peacefully.
My experience told me that most cats do know they are not feeling well and unable to do things they used to do. (jumping, running, playing, eating, and drinking etc.) However, I don't think cats know the concept of "death". Most did wanted to be left alone.
After seeing some of my cats suffered for the last few weeks /days
寵物安樂死 is probably a better option for some of them. The one with bone cancer had to suffer 4 weeks of gradually losing her friends (her siblings and friend didn't recognized her because she smelled different from the cancer), appetite and mobility. We made her a bed in the bathtub during the last two week of her life. She only allowed my wife to feed and administer pain killer to her because she didn't recognize anyone of us.
自然去 is ideal but not possible in some cases.
I think 安樂死 is a better choice if the chance of survival is slim or the quality of life will change drastically. Our cat that suffered a stroke and forgot who she was ( she was the Queen and all other cats was afraid of her) had to hide in the basement washroom in fear for 3 months then she was constantly attacked by cats that she used to bully (karma?)for another 3 months before she passed away.