No no no. You’re wrong.
You did not fail him. And while I’m sure he was stressed, he had absolutely no concept of death. Whatever fight he was putting up was based on the same behavioral trait that triggered him to try to bite you.
You did the right thing for him because that would have been his experience every time he went to the vet or to a groomer, or been asked to do anything he didn’t want to do. And it would have been miserable for him and dangerous for everyone who had to handle him.
This was predictable and I’m sorry I didn’t think to recommend that you let the trainer handle it, and that no one thought to ask you to step back sooner.
I’m so sorry that you had to experience that but whatever behavior he was exhibiting is further proof that you did the right thing.
I’m so sorry for your loss, I’m sorry that he had whatever problem he had, but you need to know that you did the right thing.
Big hugs.