What prevents venomous animals from being killed by their own venom?

The reason that snakes, scorpions or other venomous creatures don't poison themselves is because it's kept in a very special compartment in the body, specifically designed not to let the venom out and not to be sensitive to the effect of the venom. In a snake's poison glands, there are specialised cells which have genes activated which tell them how to make the cocktail of proteins (snake venom is a protein) which makes up the venom. The venom is then squirted out into a special system of ducts, lined with cells designed not to be sensitive to the venom. The venom is produced, it trickles down these ducts and into a special bag which holds it and keeps it safely away from the rest of the snake's body until it's needed.

A snake's teeth are hollow and, in the case of the cobra, curved. This means that when the snake wants to bite something, it can lock on to its prey and hook its teeth in. Muscles around the venom bag then contract, squirting the venom out through the hollow teeth and inside the tissue of the victim.

Normal tissue lacks the specialist defences found in the venom producing glands of the snake.

This is very similar to the human stomach. We make digestive juices in the stomach including acid and enzymes which could break down our body's tissues. They are stopped from doing this by the stomach's special lining, which protects it from the effects of the digestive juices.

If you were to inject cobra venom into the normal tissue of a cobra, it would have the same effect at it would on you!