You're way the heck out in the woods or desert or whatever. You see a snake. Under what possible circumstances is shooting it a good idea?
I asked that question recently, and the answer I received was that if a person got bitten by a snake, someone in the party should kill it to show the medical personnel so as to get the correct antivenin.
Nevertheless, any snake I have captured, or seen captured, was taken without a firearm. And a lot of places where one might encounter a poisonous snake are a long ways from medical care.