This ^^
This happens more often than people think it does.
also the hard stares...the freezing up...I've seen many people miss these things...and they are loud and clear warnings to me. I have a fear biter that NEVER growls...but trust me the warnings are there.
I agree with you, and Dekka. The VAST majority of people can't recognize ANY warning signals in a dog that don't involve obvious snarling/growling. I work with people, at a Pet Hotel, who didn't believe me when I said a dog licking their lips is stressed... and who often mistake a dog showing their teeth in warning as a "smile". It's shocking, really.
There are so many sketchy aspects of this situation, from the dog being tied up in a warehouse, to leaving an aggressive dog alone with a complete stranger, to the OP using positive reinforcement only "for so long" because this dog "got bigger".
If anything, it makes much more sense to use natural motivators to condition and train a dog that is large and powerful, because he's obviously winning the battle of wills.
It would be irresponsible for anyone on the internet to tell you whether or not your dog deserves to live or die - and it's your decision to make, as your dog is your responsibility! It truly sounds like this dog was set up for failure, and from a breeder who is going to put the dog they produced to death sight unseen? It makes me sad. Please, please, please get into contact with a reputable, certified, positive-ONLY behaviorist and get a professional first-hand opinion.