Why is brutality so important?

Let’s take a look at the National Socialism.
Some people mean that World War II was preprogrammed in the “peace contracts” of World War I. Ferdinand Foch a French marshal said after the WWI contracts: “This is not peace; this is a cease-fire for twenty years”.
But why the brutality of the Holocaust, why the absolutely brutal and unnecessary bombings in Poland e.g., and why this absolutely crazy warfare?

Or the Imperial Japan:
Japan was forced to open the country by the USA to the world in the Protected content but the USA didn’t colonialize the country. And then came the period of the Meiji-Restauration in the Protected content . Japan then had a huge transformation and made the step into the community of the industrialized countries.
In the first part of the 20th century it came to the brutal territorial colonialization of many countries by the Japanese Imperial Army, for example Manchuria and other countries.
Why couldn’t they also, maybe not following mulish the USA, but by following the new signs of time, or following a human wisdom, help with their immense military power, e.g. for the Chinese to end their inner conflicts and establish a civil government? The same for other south Asian countries, why couldn’t they help them to make themselves free from the colonialist lordships, beginning e.g. from Vietnam to India! Nooooooooo, they decided to be a brutal colonialist power.
(Btw, I really appreciate the Japanese culture!)

The Islamic State:
Why didn’t the people of the IS use this power vacuum, their own power and organizational capabilities to establish together, eventually with the Curds, a new Sunni-Arab state – I think there are mostly Sunni people in this region - in the northern of Iraq and Syria, or at least an autonomous region? Noooooooo, they had/have to be brutal, they have to enslave, to rape, to crucify, to behead people, to torture, and begin a war which they can never win, I think.
(I have really nothing against today’s Syria or Iraq; if they remain as they are I am also satisfied)

Why the tendency to brutality? It is JUST another little bit different way of thinking! Or are they the negative emotions, which destroy the natural way of thinking, and disable empathy, compassion and creativity?

Have a good time,

Endre