Ants can be vicious, organized, and terrifying. But while some will attack any threat that comes in their path--people included--much of their cruelest behavior is reserved for other species of ants.
Take slavery: Protomognathus americanus ants raid the colonies of smaller ants called Temnothorax, steals their children, carries them back to the invaders' colony, and forces them into a life of servitude caring for the young.
Temnothorax ants aren't the only species that can be enslaved by a stronger competitor, but they are the only ones seen launching a slave rebellion. Temnothroax prisoners have been observed slaughtering some of the Protomognathus ants they'd been charged with babysitting.