Psychologically speaking, what causes a person's train of thought to resort to killing for fun?

Back in 2013, two teenagers from Oklahoma where charged with the murder of a 22 year old Australian baseball player. What was their reason? They only reason they gave was that they were bored. It was not motivated on religious grounds, cultural backgrounds or even the basis of being territorial. No. It was just because these teens were bored.

I remember reading this story three years ago and remember how much it shocked me. And made me wonder what could possibly be going through anyone’s mind to murder someone for such an insignificant reason.

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