These are two you should take care to never kill - speckled and prairie king snakes. They are very aggressive towards other snakes, and kill venomous snakes whenever the opportunity arises. They are not as affected by the venom from North America’s venomous snakes as other animals, which is why they’re so successful at curbing the population. I have actually considered buying a bunch to let loose on my property. They’re docile and stay out of the way of humans, it seems like a win-win.