I imagine that while it doesn't need to be this vicious for humans, they have evolved this to deal with other large animals. Humans tend to avoid getting bitten so we haven't evolved the defences other animals have. We are pretty weak creatures all things considered.

Have you seen that youtube clip of a honey badger attacking a poisonous snake? He is bitten repeatedly. He falls unconscious, and later wakes up and continues eating the snake. That's the kind of predator poison gets stronger and stronger for.