Many snakes produce only small quantities of weak poison that is just adequate for their various small prey. But other snakes’ venom can be deadly for large animals—including humans. This is certainly the case for the king cobra, which is the world’s largest poisonous snake and might be capable of killing an elephant with a single bite. The king cobra preys overwhelmingly on other snakes, which have developed resistance to its venom. And that’s the reason for its powerful poison: It takes more venom to bring down another snake than it does a mammal.

http://www.popsci.com/scitech/article/2008-03/evolution’s-most-effective-killer-snake-venom

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