Why do my bees turn nasty when I kill them?
The short answer is that I, too, would turn nasty if you were trying to kill me.
The longer answer is that when you do something that threatens your bees, alarm pheromone is emitted into the air by the endangered bees as they attempt to sting you. The pheromone, which is a mixture of highly volatile chemical compounds, disperses quickly and alerts other bees in the colony that danger is near. In response, guard bees will arrive on the scene to fend off the attacker (you) by head-butting, chasing, and stinging.