How does venom kill people or animals?

There are basically two kinds of snake venom, hemotoxins and neurotoxins. Hemotoxins destroy red blood cells and tissues; neurotoxins attack the central nervous system. Crotalids (rattlesnakes) have hemotoxins, but a few (Mohave rattlesnakes and hybrid rattlesnakes resulting from in-breeding with Mohave rattlesnakes) have both hemotoxins and neurotoxins. Moccasins (cottonmouths and copperheads) have only hemotoxins. Coral snakes have neurotoxins, as to non-U.S. snakes such as cobras, Kraits, and other "elapids". Hemotoxins kill by destroying tissue vital to the preservation of human life. Neurotoxins kill by paralyzing the nerves controlling the diaphram; the victim suffocates as a result.

Very few people in the United States die as a result of envenomated snake bites -- about a dozen or so per year. In India and other Asian countries, the number of deaths is much, much higher, in the tens of thousands per year.