Most of the vets I’ve worked for will refuse “convenience euthanasias” (and will try to help the owners re-home, if possible — we got two hospital cats that way) but there will always be shady ones that want to make a buck, unfortunately. But 12 isn’t even that old. My parents’ cat just passed (was euthanized) at around 18 (we guesstimated her at 8 months when we found her). I recognize that’s quite old, but I see it happen quite a lot! My own old man cat is 16 now. Cancer and heart disease and he’s ^{knock on wood} doing great. Twelve is just not old enough to say “well what are you gonna do, he’s old!” He likely had some undiagnosed medical problem(s) that might’ve been fixed if he had been evaluated by a vet instead of just killed and tossed aside. How awful. I don’t think I could ever forgive someone or keep them in my life if they had done that. I’m sorry, OP. There’s an asshole in this story but it’s 100% definitely not you.

NTA.