It's not like he attacked and killed a child maliciously - he got in a fight with another horse ONCE.
Sure glad your vet doesn't know about my attack draft horse
I know this isn't the help you are looking for - I will leave that part alone now. You know what's best for you. Like others said, it is way better that he is going to be PTS versus being rehomed with someone who may not do well by him - I agree completely there.
I am a bit shocked at the comments from the vet though - only because I have not heard a comment like that before FROM a vet.... especially about a horse that has shown some aggression to another horse. That's all.
Big hugs to you during this difficult time.