Humans usually become murders because the Way that they were raised usually they become murderers because they have a family history that isn't so good bike say they only killed women it's most likely caused because their mother was a very bad person to them or say that they only kill boys it's because her dad was a very bad person to them
One reason is a traumatic past that caused the person to lose it. Another reason is a person could have a mental disorder, or they just plain want revenge for something. There are many reasons to how a person can become a murder.
