Why would bees evolve to have different levels of agression?

Take, for example, the difference between European honeybees (EHB) and the subset of Africanized Honey Bee (AFB). Known colloquially as “killer bees”, this hybrid species was created by the accidental interbreeding of European stock with the African variety Apis mellifera scutellata. Though very industrious, a. m. scutellata are also far more aggressive. When 26 scutellata queens and innumerable drones were accidentally released from an experimental breeding program in Brazil in 1957, they interbred with the EHB population and created a hybrid species of industrious bees that thrive in the tropical climate, but also retained the more aggressive traits from their African heritage. (Ruttner, 88)  

EFB vs AFB

Africanized honeybees are not significantly physiologically different from their gentler relatives- a single bee does not sting more (they cannot! See Mechanism), nor is their venom more potent. Simply, many more bees will sting. According to a study (Collins,1985) showed that AHB both produce more alarm pheromone(iso-pentyl acetate), and are more sensitive to it.  The threshold for stinging response in Africanized honeybees is also much lower; minor disturbances such as a slight motion, vibration, or odor are sufficient to trigger attack.  This study also showed that Africanized honeybees respond 2.4 times faster to alarm pheromone and about 30 times as fast to a moving target. (Collins, 1985).

Once Africanized bees have been roused, they are much more likely to respond in group attacks, focusing on any moving target, and often pursuing targets as far as a kilometer. (Winston 1992). 

It has been theorized that the rapid defense response in AFB's is a result of adaption to life in tropical climates, where there are more predators that prey on honey bee colonies. Since this higher attack rate would favor evolution of bee colonies that responded most rapidly and aggressively, the bees in this tropical climate became far more agressive. (Voeller, et al. )

It is important to note that widespread domestication of bees has lead to a larger population of relativly docile bees. Beekeepers make an effort to kill exceptionatly agressive queens, and to foster calmer populations. As a result, the behavior of domesticated bees is representative mostly of those populations where humans take an active roll in bee husbandry. Occasionally, more agressive strains (such as the Africanized bees) will take over or outcompete the more passive bees. Thus, bee behavior is truly a product not only of the species' history, but also of their co-existance with humans.