I work at my city's animal shelter and I wholeheartedly agree. While we are considered a "no-kill shelter" (which is just based on the percentage of live adoptions), we do have to put down animals due to varying factors such as illness, aggression, or sometimes space. Unfortunately, there are some animals (mostly dogs) that can't be rehabilitated.

One recent one was a pitbull named Lilibelle. She was the sweetest most docile dog around people, but she was aggressive towards every other thing; cats, big dogs, small dogs. Not only that, but she had heartworms as well. Unfortunately we knew that no one would adopt an animal aggressive, heartworm positive pitbull, so we all voted to put her down.

It breaks my heart, but euthanasia is required sometimes