Cut its head off as swiftly as possible would be the most humane way i would imagine. If it was obviously suffering i wouldnt have a problem doing it myself. If i wasnt sure then i would take it to a vet. If they thought the best option would be to cull it then i would probably still do it myself at home by cutting off its head. A needle to the heart sounds wayyyyy more painful than a quick guillotine type execution imo.
I remember reading somewhere that after decapitation scientists estimate humans still remain alive for a second or two. Idk if thats true or not or how it works with snakes though. Or how anyone would know that lol...?