Why must both constricting and posionous snakes eat their prey alive?

There is a very common misperception among people, even among many snake hobbyists, that snakes must have live prey.

Explanation:

This is not usually true. In the wild, snakes, like most other carnivores, will usually take any opportunity to get a free and easy meal, and will not pass up a freshly killed prey animal if they happen upon it. In fact, snakes have been known to eat a fair amount of carrion which has been dead for some time, and the stomach contents of wild snakes often contain prey that must have been in an advanced state of decomposition when it was eaten. Thus, although your snake may prefer to eat live prey, which it kills itself through constriction, this is not at all necessary for the health of the snake. In fact, it is best not to feed your captive snakes live prey at all.

on island x the species needed to adapt because they were being subject to predators which didn't inhabit island y.