I agree with it under the right circumstances.
We had a dog named Snuggles that was our baby, our first pup. Snuggles was part of the family, and we called her a little furry human. She had kidney failure and would not eat anything or do anything, it was hard to watch her suffer. I had to make the decision to put her to sleep and it was the hardest thing I've ever done in my life. I look back now and know I did the right thing. Let her be peaceful rather than to suffer anymore.
I don't agree with people deciding they don't want an animal and then deciding to euthanize it. That's not how it should work, even though it does happen. Maybe there would be less euthanization of animals if people would get their pets spayed or neutered.
Ed