Each animal's venom is a different mix of toxins - a complex chemical cocktail that causes a biochemical reaction in many victims. Depending on the mix, the victim may be paralyzed or killed by heart failure, suffocation, hemorrhaging, or other causes.
Neurotoxins attack the nervous system, cause muscle paralysis, and shut down respiratory systems.
Hemotoxins attack the blood, destroy tissues, cause hemorrhaging, and reduce blood pressure. Cardiotoxins affect the heart directly. An estimated 25 percent of snake bites are dry, that is, they don't inject venom into the wound. Once an animal has used its supply of venom, it's essentially defenseless and no longer has the ability to obtain its prey.