I've only had to have one pet put down...and I had to put down one pet myself. In the first case, it was because it got violent, in the second because it got horribly mangled and there was nothing that I, or anyone else could do.
There are rays of hope in the world of SPCA...the one that runs a chunk of the South Shore of Montreal has a no-kill policy. They will keep the animal until it gets a new home.
Thanks for bringing up a hard but tough topic though...but remind me to hold you down when Prof shows up with a dirty-stick.