Unfortunately it appears children are actually more likely to do this sort of thing than adults. Like all animals of prey humans are born cruel, or rather without empathy, because it used to be an essential "quality" required for our survival in the kill-or-be-killed prehistoric world. Little kids for example usually have no misgivings about maiming or mutilating insects in the most cruel fashion until proper socialization and the development of empathy restricts this behavior to the relative necessity of killing insects or even animals (more rarely nowadays) without inflicting more suffering than necessary, and this often evolves in adults preferring to shoo insects outdoors rather than hurt them whenever possible. If I were a fly I'd much rather be let out than swatted but if I had to die I'd still much prefer swatting over having my legs and wings pulled out. Empathy is the ability for a human to think this way and is so ingrained in modern societies that soldiers must be "reprogrammed" to act more like primitive humans because unlike even relatively recent history (medieval times), soldiers have not be raised as such.
Obviously an 11-year-old who puts razor blades on slides displays a lack of empathy that is more characteristic of much younger children who, thankfully, lack the imagination or physical ability to conceive of such evil plans due to their young age, and usually develop a conscience and sense of guilt long before they can act on barbaric impulses. I think this kid might be a sociopath but it's hard to tell without knowing how he is reacting to the event. Is he enjoying the victim's pain, or does he feel guilty about it? If it's the latter there is hope, but if it's the former...
