A large part of the human brain, just like the brain of other animals, is devoted to threat detection. These circuits are constantly evaluating our internal and external state for threats. This cannot be conscious, because that’s too slow. It’s deep in the brain below the cerebral cortex, where consciousness arises, in a region called the hypothalamic attack region.
The hypothalamus is where a lot of our urges and automatic functions are carried out, like sexual behavior. The hypothalamic attack region controls defensive-aggressive behavior. If scientists stimulate these neurons with an electrode, an animal will instantly become aggressive and attack a test animal in the cage.
Source: Your Brain Is Hardwired to Snap