Hugs to you Dear, for your pain and loss, and for doing the "right" thing! You saved him from more suffering. He would have probably ended up with the same fate, but with so much more suffering before it happened. Its is so much better to let them go and save them from the pain and suffering that they would have gone through waiting for nature to take its course.
I too am feeling what you are right now, Just this week we had to let my basset hound go to the rainbow bridge. We adopted her as a puppy and she had been our friend and constant companion for the last 13 years. She was having problems this past weekend, have non stop anxiety attacks, and when she did finally rest from exhaustion she was almost in a coma like state. We could have had a bunch of test run, to find out exactly what was going on, or put her on anti anxiety drugs for the rest of her life, (but they pretty much knock her out and make her sleep) so what kinda life is that. We did her the favor of saving her from anymore pain or suffering.
My heart goes out to you, Remember him, cherish his memory, Take his halter or something else that was his and make a memory keeper out of it. He will always be with you in spirit, and he will be waiting for you over the rainbow bridge, and will probably thank you at that time.