How do I put down an aggressive dog? My vet won't do it and no shelters will take him. I'm at my witts end.

My ex g/f abandoned her aggressive dog in my apartment. I've been watching him for a year. He's bitten my friends. I rehomed him, but he bit the last owner. I called shelter but they're all full. I called my two vets and they won't put dogs down for aggression.

I've tried to do right by him but I'm at the end of my rope. I've done the Instinct behaviorist and spent 5k on him. I'm no longer willing to spend another dime on him. THIS ISN'T EVEN MY DOG!

Any recommendations, I tried to do right by him? I feel like I'm being backed into a corner with no options but release.