As a person who works in the field, and has had to triage own pets during similar events, you absolutely did the best you could and made the best choice. It will still hurt and break your heart. That’s normal and shows you’re a very caring person. These are definitely common feelings after events like these.
You showed him mercy, love and care. We can only hope that us as humans are able to be shown the same mercy from suffering if the day even arises.
Many hugs to you