When kipling says that the reward for imperialism is “the blame of those ye better/ and the hate of those ye guard” to what is he referring to? what would those natives who imperialist powers “bettered” blame imperial powers for?

He is referring to the policies of colonialism that were widespread. He was British and visited many of the British colonies, but spent a lot of his time in India. These were all lands that Britain had previously colonized and he is referring to the colonization that was predominantly done by white people from Europe.

They would blame them because they wouldn't be free. The colonies were getting "bettered" because they got technology and similar things, but they were blamed because the colonists didn't have independence and freedom and were stripped of their resources and natural wealth which was common at the time.