You know I've struggled with this too. That's the burden of being human, is we apply our concepts of morality to the rest of the world as we see it. The rub is, this type of activity happens every minute throughout the world in nature and we never see it. We as human beings do not like to see pain or suffering, when it is a natural part of the cycle of living and being.

My $0.02 USD.

It sounds like it's beyond saving.

If it were me, I'd put it on a shovel, carry it out back behind our fence where the neighbors couldn't see (so they wouldn't get alarmed), and shoot it with one of my BB guns to put it out of it's misery.

If you whack it with a mallet, then you're going to have to wash that mallet really good (if it's a kitchen mallet). If it's a tool, then it's going to get messy real quick. Your best bet would be to get some kind of garden implement with a sharp edge and a long handle, like a shovel or something that can cut cleanly with enough force. The goal is to give the poor lil bugger a quick, clean death.

If you can't or won't do any of the above, then I wouldn't worry about it even though you probably will. You need to remember that this is nature at work, and nature does not understand the concepts of cruelty, malice, etc. Nature just is. It's been there before us, and it'll be there long after will kill ourselves off with nuclear weapons.

In short, whatever choice you make will be one that you can and should live it.