Killing

Why do we resort to killing? Why do our brains think of murder when we are angry, mad, or sad? This had to have happened to someone then carried this feeling on to others. One of our feelings and moods is just killing. Some think about it all the time - some don't. Why? Is it because your family has thought about murder multiple times that it's just natural in the family? Or maybe your family is all goody-two-shoes.... But that's just a theory, A GAE THEORY