don't have the animal put down... if you really look at it as part of your family... find out what's wrong with it and get it treated... that simple....
if your parents or sister had kidney failure, you wouldn't take them out back and shoot them.... so why would you with your dog? Now if you don't think of it as part of your family, take it out back, play catch with it.... and then when it's bringing back the ball shoot it between the eyes.