That being said, I'm sympathetic to your view that it's a matter of morality or one's upbringing. Plus, most learned people tend to refrain from doing this since they are repulsed by such depraved behavior-by which I mean they strive to be bright minds instead of uncouth souls.
I'm not a shrink nor a sleuth, so I don't know much about serial killers, despite having read some crimes novels featuring tough-cookies and gangsters. And I find that some characters in these novels treat their beloved ones nicely, even though they bilk their clients and even put away their enemies without showing mercy; some of them even have cats or dogs.
I am actually not saying that it is how you are brought up - if someone is a crazy enough to be a mass murderer it doesn't matter how you were brought up. What I am saying as a child doing something cruel - is because a child does not fully understand the repercussions - that they fully understand the harm of their act.
I have a personal experience. When my children were both very young - one was an infant about 6 months or so and my other daughter was 4, they were in their car seats in the back seat. The baby was crying and I was almost home.
When I got home, I took the baby out of the seat - her whole face had scratch marks going down her face and was bleeding. I looked at my 4 year old in horror! How could my toddler do something so cruel - I immediately called the doctor described what happened -- the doctor assured me the baby would be fine, just clean the cuts and they will heal quickly.
Then I asked what do I do about my 4 year old ? How could she do this? The doctor calmly explained as she could hear my 4 year old screaming and crying in the background - she is 4 years old, she probably heard the baby crying it was bothering her so she scratched the baby. A 4 year old does not understand the repercussions of her actions. They just react. They need to learn this - and it sounds like she understands now.
My daughter is 21 now she did not grow up harming animals or murdering others. Quite the opposite -
A crazy person does not have the same morals and does not learn and understand that this is cruel even with proper upbringing.