I don't think there is a right answer. Use your best judgement and do what you feel morally comfortable doing. Some people like to help, some people end the misery. Neither is wrong.
You did the best you could. Honestly, this was probably the best case scenario. He's free from pain, and you didn't have to make the difficult decision to cull him. It's never, ever easy to be in this situation. I'm sorry for your loss. <3