What makes someone a serial killer, and do they have to commit a certain number of killings to be considered one?

A serial killer is typically someone who kills three or more people over a period of time. They usually take breaks between the murders. For them, each killing is a separate event and is often driven by a desire for excitement or pleasure. Serial killers often don't feel bad about what they've done and can become very focused on themselves. Some of them are classified as psychopaths because they lack empathy and guilt. They can also be very good at hiding their true nature and might seem completely normal and even charming to others.