Can we please stay on-topic here folks?
Only because you asked so nicely.
1) Is it ok to release unwanted/unhealthy Cornsnakes in your backyard?
Can you guarantee it will never leave your yard?? If so, go ahead. If it's an unwanted, healthy, normal or Okeetee (with no hets), released into their natural habitat...then I can't argue against it. Any other circumstances would be a big NO.
Aside from the valid environmental arguments, take a moment to think of the herper that lives not far away. They find a corn in their home or yard thinking it's an escaped pet. They take it in and spend their time, money and emotions trying to care for it only to have it end up being a bad experience.
Think of the people living nearby who don't know jack about reptiles. They find a snake in their yard/home and totally freak out because it doesn't look like snakes they are used to seeing. Next thing you know we have some more great press for our hobby that we love so much and (if we are really lucky) the press will educate us on how "corn snakes are not poisonous."
2) Should you euthanize or let nature decide by doing nothing for sick/deformed/refusing to eat snakes?
Sick? No, take it to the vet. Can’t afford it? Try cutting extra expenses like the internet…
Deformed? Yes and no. A deformity that will not affect long term health I’ll leave up to the breeder, otherwise yes.
Not eating? If it’s in bad shape and you have tried everything, yes, end its suffering.