I think the Boers treated the Africans so cruelly because they learned it from the British. The British did the same with them. Therefore, there was no question of any other sparing methods, they acted in a proven way. As for the passage as a whole, we can conclude that the leader of the South African tribe in despair, he asks for help from the British, because he no longer has anyone to turn to. He wants peace for his tribe, and he no longer has the strength to fight the enemies, since almost all of his warriors were killed by the Boers.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.