I do not think that children are very aggressive. They just grow up and, unknowingly, make mistakes.
It seems to me that childish aggression is a cliché associated with the fact that a small child does not know what death, loss, separation are - he just has not experienced these feelings in full, his brain has not learned them, does not understand what to do with them. This is a small, inexperienced, strong organism that needs to gain many different impressions as quickly as possible, assert itself, survive, and figure out what's what. Until the psyche is formed - until the age of 20 - a child and adolescent copes with a terrible and difficult experience much easier - sometimes it is really forgotten, goes into the subconscious. It may appear years later, but here and now, in childhood, the brain is not yet subject to reflection, it just lives and survives. The child feels formal pity for the "consequences of bad actions", but only an adult who has seen death with his own eyes, bored, lonely, can sincerely lament, worry, love - his psyche has grown enough to experience these complex feelings.
A child who, from an early age, is instilled with a feeling of "irreversibility, irreversibility" of certain actions and does not protect against pain and loss, according to my observations, psychologically grows up earlier than his peers: reading stories about Seton-Thompson animals, explaining what death, illness, destruction is very important. The need to take care of pets, their humanization helps.
Did you break your favorite toy? - I'm sorry, it cannot be repaired, it "died", it will no longer be there.
Here it is important, of course, not to overdo it, because with children you will not always understand whether they really felt sorry and sad, or they just imitate the feelings that you want to achieve from them.
However anyway, the legs of the flies cut off everything. It's good if the parents notice this and then the child is limited to flies only.
Apparently this is due to the lack of development of the "Super-I", which is responsible for moral, ethical and moral barriers. Children, being not limited by the developed "Super-I", are guided only by the desires and aspirations of their "Ego" ("I"), which is not too burdened with feelings of guilt and compassion.
However, jurisprudence believes that by virtue of children's lack of complete assimilation and acceptance of such social values as morality, morality and ethics, the latter are not able to fully realize the criminal nature of their actions, and therefore be responsible for them.
Our children, in general, are shielded from all violence by our guardianship, and therefore they cannot realize the degree of suffering that they can cause to others. A person who has experienced suffering will think twice before causing this suffering to another person.
As far as I know, the cruelty of adolescents and children is associated with several factors:
1) Strong dependence on social status and place in the school hierarchy. Humiliate or humiliate you.
2) The "rampage of hormones" and crowded (and often inappropriate) team, in which teenagers are forced to be, lead to an increase in aggression.
3) Immaturity psyche.
UPD: Yesterday I watched this video, where the question is revealed. The main reason is point 1)