This sentence jumped out at me. You don't think that aggression is a valid reason for euthanasia? What if the owners can't afford a specialist? IMO, by saying that, you're punishing the dog even more so than you are the owners. It's not always black and white, and honestly, I would rather see the dog euthed than live a miserable aggressive life with owners who are either unable or unwilling to pursue different options. What if you decline to euth, and they DONT go elsewhere? What if they just lock the dog in a crate and proceed to neglect him because "they didn't want him anymore, anyway." Or they put an ad on craigslist, and send him home with first unsuspecting person. That person can't handle him, and rehomes again, and again, and again. Who loses? the dog.
I'm certainly not a supporter of "convenience euthanasias" of perfectly healthy and sane animals, but this is a whole different ballgame.