First of all, this is a colony.

We don't yet fully understand altruistic behavior in terms of evolutionary biology, but when you are discussing a colony/hive social structure as with ants/termites/bees the colony itself, for all intents and purposes, can be treated as an organism in and of itself.

IF you can accept that these workers/soldiers themselves act as organs within a larger structure, you begin to realize why these individual critters are led autonomously by pheromones and instincts to defend their hive and, if need be, sacrifice themselves. They do so in much the same way that your body begins to shut down the organs that are not vital to your survival in the event that starvation sets in.

Remember, regardless of how stupid such an adaptation may look to us, it has led these creatures to be extremely successful. Hell, ONE ant colony essentially holds dominion over most of the Mediterranean.

That stinger had to evolve from somewhere and the cheif rule of evolution is that it can only build upon what it already has to work with. In the case of the common honeybee, it would appear that, for at least the last few millennia, having a stinger renders an evolutionary advantage that outweighs the cost of sacrificing those who sting.

Note that this is not the case with, say, hornets or wasps, which have managed to attain a similar evolutionary trait without that glaring drawback.

A great deal can be sacrificed for the good of the colony. This is what bees and ants do-- and they do it very well. They have for a long time now.