this is a topic close to my heart.
I recently decided to put my young bully girl down for killing cats and harrasing/maiming livestock (she had been caught on two seprate occasions with live chickens in her mouth that she dropped when commanded, and she pushed though my daughter into her goat pen and playfully shook her billy goat until I got there and commanded her to OUT the nail in the coffin was when she broke out of a reinforced crate and killed 3 cats of three seperate occasions within a week.
She was raised VERY well, perfect recall and GREAT drop it/OUT. Great off lead, just an extremely high prey drive and she would seriously feed off any other dogs behavior, she was an accident waiting to happen. I was heart broken, but she was not a dog that I was comfortable rehoming as she had become incontainable in a crate or fence unless she wanted to stay put >:( as well as her predatory habits, and with small children it was just an accident in management waiting to happen, so I made the call. It sucks and I miss her every day, but better a day too soon than a moment too late.
If I did not have kids that increase the incidence of a management breakdown, I would have kept her and managed the situation. Good luck CDE driver. It's a tough spot.
I admit, I have reading comp. problems..sorry
I have reading comp problems..sorry, just reread the OP. I missed the part about this happening at the barn (just saw the feral cat colony part).
OP....I do not think you should put your dog down..BUT, I would never take him to the barn again if the barn owner has cats and he has now killed one.
What did the barn owner say? I would be surprised if she allowed you to bring him back.
Yeah, I would keep him at home and exercise him somewhere other than the barn.