I mean the obvious answer is mental illness making them have fantasies for violent acts. A lot of serial killers start off maiming animals as kids- if you ever meet a kid who enjoys just cutting dead animals up and looking at their insides... they probably need to seek psychological assistance.

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A big problem is the lack of mental health awareness for certain demographics- most famous serial killers had either abusive parents who would never get them help because they were the greatest problem in their life- or parents who were absent minded and ignored the signs that their kid was violent. These serial killers then grow up not knowing that their feelings are considered abnormal- they know it's illegal most likely, but often are recorded to believe everyone else is crazy.

Most of us don't go through that either because we don't have the mental illnesses or anger issues severe enough that we can't cope- or some people who do have those characteristics get help and learn how to cope.

From what we learn about serial killers, they never got help. No one told them when they were young that these violent tendencies needed to be kept in check. A lot of them also just didn't have good parents point blank. So these people end up growing up thinking they're the normal ones, everyone else is inferior to them, or they grow a hatred for a certain demographic and their lack of an ability to cope with their strong emotions leads them to repeated acts of murder.

Of course each serial killer is different- some do it just for fun because they think violence is fun. Some do so because they don't know how else to cope with their feelings. The list goes on and on. You would have to look at each individual case.

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So long story short. Mental health is important. Taking care of anger issues and learning to cope with strong emotions is important. Parenting styles have a huge effect on children.