I have had to do it a few times. Doesn't really bother me but then again they were not my dogs and they all were suffering from something. One was a big rot mix that was a stray and the dog warden sad just shoot it the next time so I did.
That brings to mind one of the worst gun stories I have ever heard. I remember a teacher in high school that had no business looking at a picture of a gun (in a school where the hillbillies kept shotguns in their car, and yes not that long ago) telling us that he had to put the family beagle down...said he dug a hole and grabbed one of his dad's gun...said .22...and then some other numbers on it... I think any dog deserves better than a poor shot and multiple rounds of .22-250 at close range...
I could no more shoot one of my dogs than I could shoot my wife or sons! I spent $2,000.00 on a Teacup Poodle and it was the hardest thing I ever did when I dup Muffin's grave. Family is family! Some one else's dog, NOPE! I'm a confirmed dog lover.