The harsh conditions of the Great Plains meant that both the new settlers and the Native Americans had to struggle to survive, and they fought hard against anyone who threatened their way of life.
There was little understanding between the various sides in the conflict, making it hard to distinguish between 'goodies' and 'baddies'.
There a number of ways you can look at the conflict on the Plains:
It is possible to see the conflict as a clash of cultures. White Americans did not understand the Native Americans' way of life. Consequently, they distrusted and feared them, and could believe anything (including torture and deceit) of a people they did not understand. Conversely, the Native Americans felt that white Americans were devils who ruined the earth. Differences of culture caused them to hate and despise each other, and led to war.
The wars might be seen as the result of racism. The white settlers believed that the Native Americans were inferior. They felt justified in saying that 'complete extermination is our motto', and in slaughtering the buffalo to starve the Native Americans to death. In 1864, Colonel Chivington justified the massacre at Sand Creek by saying:
Kill them all, big and little: nits make lice.
Faced by an attitude of genocide, Native Americans had nothing to lose - as the Sioux Chief Gall said:
You fought me and I had to fight back.
It could be argued that war broke out simply because the white men wanted the Great Plains - firstly to cross, then for gold, then for cattle and then for farming. Many white Americans believed that it was their manifest destiny to take over the Plains. They took the land that Native Americans believed belonged to everyone.