Why queen bee kill the worker bee?

A new queen can actually be raised from any female egg. Whether an egg develops into a worker or a queen depends on how the larva is fed. From hatching, all larvae are fed on a highly nutritious substance, called bee milk or royal jelly, produced from special glands in worker bees for two or three days. Workers (and drones -- the males) are then weaned off on to a mixture of honey and pollen. But if the larva is destined to become a queen it will continue to be fed on royal jelly until it pupates. The richness of the diet trips developmental 'triggers' and the larva develops into a queen rather than a worker.

One other difference: the workers will build a special 'queen cell' around the developing queen larva. This is much larger than the normal worker cell and stands proud of the comb.

Should a queen die suddenly. the workers will simply 'promote' a selected worker egg and develop it into a new queen.