I have lost two beloved pets to old age, a petite tuxedo cat named Cleo, who lived to 17 and a 100 pound husky/golden/german Shepard mix named Allie who lived to age 14. I had them both put to sleep when they began to suffer, I held them both in my arms while it was done, it's as close as you can get to a painless death, I just wish they did this for people in more places.
Yes, nature can be cruel, to pretend otherwise is to be naive, but usually that death is quick, it's only when people get involved that real suffering happens. I have no tolerance for those who would adopt a pet and not take responsibility for it, or worse, deliberately make an animal suffer.
My current cat is a bit of a hellion, she has been known to steal things and will wake us up with body slams if the food dish gets low, not pleasant at 6:00 am.. But I love her just a much as the others, if not more. She is wicked smart and quite the character and makes me laugh every day, I think she knows that is her job. If she isn't sneaking up behind me to smack me on the head she is waiting under the bed to grab my ankles. She also makes up games to play to keep herself entertained. I kept seeing her drape herself over the back of the chair and wiggle around. It took me a year to figure out she was trying to grab her tail from between the slats, lol. I have found her at the top of the stairs swinging things in her mouth to build up momentum then flinging them down the stairs to chase.