What is the most humane way to euthanize a reptile?

This is sort of a dark topic I know, but there comes a time in a keepers/breeders life where they will eventually have to make this decision.
When an animal cannot care for itself or is suffering from a disease or has sustained a life threatening injury and is only getting worse, a lot of people believe that it's best to just let that animal rest in peace rather then go through life suffering. I had a baby Savannah monitor a couple years back that had the weirdest disease where it eventually needed help to eat, drink, and defecate. Pretty much turned into a lifeless vegetable. He had actually drowned in the sink during a soak one day (during a manual defecation session) and I brought him back with CPR but after that it seemed that he no longer had the desire to live. He had the weirdest blank look in his eyes, didn't bask, eat, or do anything on his own. We kept him alive for another week or two after that day then we decided it was best to just let him go. We made him a little coffin, lined it with plants and flowers, said goodbye as we put the lid on and into the freezer he went. Sad as it was it was the best thing we could've done for the poor guy. We weren't about to let him starve to death or continue to keep him alive against his will. Two years later I face the same predicament. I just hatched a baby leopard gecko that has a severe neurological disorder that prevents him from walking and balancing. He is about a week old now and has shown no sign of improvement. I have manually fed him the last couple days to try and encourage development, but I don't think it's going to happen. Today he was "lost" in the lid of a pickle jar totally disoriented and I felt really bad for him.
I don't want to manually feed him every day because he's too discombobulated to hunt on his own, and I don't want him to starve either.
It's coming down to that point of judgment again and I want to be sure that if I have to do the duty that it's done right and with as little discomfort as possible. I understand the quickest most painless way but I don't really want to deal with a horrifying memory of being the executioner. Things like that stick in my mind and arise at the wost times possible.