Speaking with a Park Ranger in the Grand Canyon some years ago, he said roughly 90% of the people who get bit by poisonous snakes know the snake is there and don't avoid it. They're either toying with it, trying to take it's picture, or otherwise getting in its' way.
How to properly kill a rattlesnake? Shoot it from a safe distance works for me.
You can never have too much ammo — unless you're swimming.