Children simply do not have the capacity for empathy until a certain age. They can understand rules, and even understand why the rules are good at a surprisingly early age (recent research shows this starts around 2 years old). They know "Do not do X because it might hurt the other kid" but they do not have the capacity to imagine themselves as the kid and understand the kind of hurt they can inflict.
Teenagers are a different story. Teenagers in our society are so viciously abused by their parents and all of society that they're simply lashing out. Every aspect of their life is controlled, their maturation is thwarted at every step, every thing they read, watch, or play is heavily censored, etc. At the beginning of adolescence, people gain the capacities of abstract reasoning and the emotional centers of their brain become fully active. It is important to understand, this means much more active than adults. Teens feel things more intensely than adults do because they have not learned yet to deal with things. They CANNOT learn how to deal with emotional situations until they actually do deal with them. They have their existential moment, the moment when they realize that they and everyone they know will die one day, and it is usually very disturbing. Instead of being helped in this time of need, they are fought against by parents and society. They naturally pull away from their family in terms of physical affection and are supposed to fill their need for affection through their peers, but the structure of our society and the severe sheltering forced upon them by unthinking, uneducated parents prevents it. So they function in a constant state of frustration and a starvation of affection. When they hear a news story about hundreds of people dying, and then observe all the adults ignoring it, it cuts them deeply. Their reactions are various, but all involve suffering. The ability for critical thought is within their grasp and they are desperate to learn everything about the world... but we stop them, and put up countless barriers to keep them ignorant. We lie to them, regulate irrelevant points of their life, and refuse to acknowledge their capacities.
Keep in mind that for hundreds of thousands of years, adolescents were the ones starting families. They were the ones going out and getting a job and supporting their family and making life and death decisions. Now, we pretend that they are children. The human brain evolved so that the experiences they are driven to seek out would cause maturation and development. In as far as they are thwarted in this is how far their maturation is prevented.
If you want to know why they are angry and cruel, imagine what would happen to you if everyone around you started insisting that you are incapable - of everything. Imagine that your workplace instituted the rules of high school. No talking, no asking questions, no going to the bathroom when you need to, no helping each other, no touching, no learning what you want. Imagine every thing you were allowed to read, watch, or play was neutered and had its edges rounded, insulting you with every single line. And imagine that every tragic event in the world affects you personally. How would you deal with it, given the resources teens have?
