I don't just say that I agree with this, I have lived it. I euthanized a sweet little dog who was a poor candidate for rescue, rather than look for a "sanctuary" to send him to. As I looked into his eyes while I pressed the syringe that would end his life, I swear he told me "I know you're not ok with this, but it's ok, because I am." And he was right.
http://notesfromadogwalker.com/2012/07/21/how-i-failed-as-a-rescuer-lessons-from-a-sanctuary/