Putting down dogs sucks, but at least you'd be doing it for a good reason.
My first dog when I was a kid (a german shepherd) was about 13 years old when it got sick, and we could tell it was on its way out. Literally the day we decided to take it to be put down, we found it passed away in the back yard.
The next dog we got was a german shepherd/doberman cross that turned agressive and unpredictable and my dad decided to put it down, it was only 2 years old. That really pissed me off.