Killer bees are properly referred to as Africanized honey bees. They can kill people, but only as a side-line. Their main business is making honey and more bees. They're dangerous because they are more aggressive than European bees. But more about that later...
In the 1950s, honey bees from Africa were cross-bred with European honey bees in southern Brazil. The hybrid was intended to be a more vigorous, productive bee; which it was. Unfortunately, the more vigorous, productive - and far more aggressive - strain escaped into the wild; and that's where our story really begins.
All honey bees occasionally swarm; that is, they pack up and move on. European honey bees swarm when they are dissatisfied with their living conditions, such as lack of food, poor sanitary conditions, or overcrowding. They are noted for being easy to please. If they have a nice hive, they tend not to want to leave it.
Africanized honey bees, on the other hand, just can't stand being where they are... wherever that is. This increased tendency to swarm, leads to two other differences in their behaviour: they are fiercely territorial, and possess the ability to brood anywhere, no matter how minimal the shelter is. Anywhere from a hole in a tree, to an overturned plant pot, to the back seat of an abandoned car.
They have been steadily migrating north, through South and Central America, and Mexico. During 1990, they reached the southern United States, and can now be found in Arizona, Texas, and southern California.
As they have moved northwards, the bees have come increasingly into contact with people and livestock. If a person is perceived to be a threat to the nest, he or she will be attacked viciously and relentlessly. The venom is no more toxic than that of European bees; but far more bees are likely to join the attack; so the victim is likely to be stung far more times. The very young, the elderly, persons in ill-health, or those otherwise unable to escape may be stung to death. Any bee sting in the mouth, throat or respiratory tract can be fatal. Since 1990, there have been 15 deaths in the United States attributed to killer bees.
Africanized honey bees look exactly the same as European honey bees to the unaided eye. The only definitive means of identification may be their method of attack. If you discover a nest of what you suspect to be killer bees, inform the authorities or a competent exterminator. Killer bees are too dangerous to be dealt with by people who are not qualified. If you are attacked, run away and hide somewhere bee-proof (a car or a building with closed windows).
As time goes by, and Africanized honey bees come into ever more contact with gentler European honey bees, the genetic programming for their violent tendencies will become diluted. We may never see the day when honey is sold in jars with pictures of snarling bee faces.