They do it because of how they're raised - or I should say, not raised by emotionally absent mentally dead crack-head mothers. Absent fathers or fathers unknown even to the mothers. Bitterness over being poverty stricken. Anger that needs expression at seeing what others have. As our population gets further and further from a normal lifestyle people have lived for thousands of generations, the worse society gets or so it appears. Too many people can't cope with the stresses of getting an education and a job paying more than subsistence wages. Or they simply can't bother to better themselves. Welfare is too easy to get so another generation of poverty stricken angry people hits the ground running.
And of course that smaller group of well educated privileged individuals who do it out of boredom and lack of morals and ethics. Look to uninterested poorly paid Nannies and wealthy parents who spend their time globe trotting or at the least also "not there" for their children.
You may as well ask why these depraved soulless reprobates set dogs and cats on fire or break into schools and churches just to destroy them for their sick gratification. I'm to the point of having no compassion for these teenagers who will become violent criminals, even homicidal as adults. They're lacking a conscience and will prey on society, prey on us for their living. You can't cure sociopaths.
Other than execution as in the old days, there doesn't seem like much we can do about these lobotomized Neanderthals. Our Justice system is a joke.... there goes my blood pressure.......