Opisthoglyphous snakes

In insects venom is used predominantly as a defensive weapon. Wasps, bees and ants use formic acid in their stings to cause a painful burning sensation that will either kill or injure their enemy enough to make them think twice about attacking them again.
Amphibians all use their venom for defense. In amphibians the venom is secreted through glands in the skin to make the animals unpalatable.

So as we can see venom is a predominantly defensive adaptation. An adaptation that has found it's way into every class of vertebrates except one. The birds; the only class to forego any venom.
In mammals we have platypi with venomous claws, then there are the fish which comprise too many venomous species to count and finally we have the reptiles.

All venomous reptiles are squamates and of them snakes make up the bulk. There are only two species of venomous squamates that are not snakes, the lizards of the genus: Heloderma. These lizards use their venom for defense as well and can deliver powerful and painful bites.
In snakes venom has found a new use, for offense. Since snake prey generally has the advantage of speed (not to say that snakes can't be speedy. A black mamba traveling at 17 mph is nothing to sneeze at.) snakes had to find a new way to take down their prey without running the risk of losing them or getting too hurt in the process. Enter venom, a fast and effective mode of subdueing prey items with minimal risk to the snake.