Hippy types will often quote the famous line that "man is the only species that kills its own." Of course, any cursory glance at Animal Planet will tell you this is complete bullshit.
But it's not just that animals are mindless savages--and they are--it's that some of them are just straight-up douchebags. Such as:
Ants are famous for hard work and being able to lift many times their body weight, making them the perfect heroes of an Ayn Rand novel, or the perfect working drone of communism depending on who's telling the story.
Why Are They Assholes?
Ants are the only species besides humans that carry out wars and enslave their own. And by enslave we mean pulling out the ant whips and putting the other guy to work against his will.
When they're not doing that, they like to tear each other limb from limb.
Less violent ants prefer to fight their battles via choreographed display fights where specially chosen ants have something like a dance-off to decide which side wins. It's basically like the movie Bring It On, except it ends with the losing team in chains or eaten (which would have improved that movie massively).
The victors raid the other nest, capture any survivors and steal all the eggs. Survivors and newborn ants are then forced to work for the victors' nest as long as they live; or at least until their new masters get a craving for some fresh slave ant, and devour them.