That looks pretty nasty, If I were to guess what has done that; I would have to say it was likely a bird that tried to make a meal out of that snake, I see snakes in the field all the time that have had their bellies pecked by birds, that is just my opinion though.

I would leave it be and let nature take its course, as was said earlier, unless you plan to make a pet of it, it can not afford the added stress of being handled right now.