Most snakes are under terrible duress, many are endangered, and when they run into a human, the human typically freaks and kills the things whether they are poisonous or not. I believe humans have instinctive behaviors, and fear of snakes is one of them. Ninety nine point nine times out of a hundred, when I have suggested to people not to kill the snake, they either kill the snake, or justify why the killed the snake, based on fear and irrationality.
According to the web 25 species of snakes in America are poisonous, but I did not find the total number of species in the US. However the same sources say 15% of snakes world wide are poisonous. I have read poisonous snakes in Africa and East Asia are the worst for toxicity. But overall, the snake you see in the grass is probably looking for mice, rats, etc, not you.