What are the main themes of The Devil and Mr. Casement by Jordan Goodman?

An additional theme is the hypocrisy that lies at the heart of the colonial project. The British government that commissioned Roger Casement to investigate human rights abuses in the Congo was the very same administration carrying out abuses of its own throughout its vast colonial empire. Unthinkingly, the British government appeared to hold to an untenable notion of "Good empire, bad empire," that there was somehow a difference between the supposedly more civilized British Empire and its Belgian counterpart. But in carrying out his extensive investigations, Casement came to realize that such a distinction is utterly bogus. The numerous abuses he uncovered represented a savage indictment of the whole policy of colonialism—British or otherwise—with its rampant exploitation and subjection of indigenous peoples.

One of the major themes in The Devil and Mr. Casement by Jordan Goodman is human rights abuse. The author tells the story of how a Peruvian rubber baron subjugated and abused local Indians in the Amazon to produce rubber. The story focuses on the human rights abuses that took place in the jungle. Roger Casement carried out the investigations courtesy of the British government and produced a report on the findings. The report was made public and led to a global outcry about the injustices taking place in the Amazon rain forest.
Another theme that is addressed in the novel is execution. After producing comprehensive reports for the British Government, Roger Casement was hanged to death because of working with Germans to make Ireland an independent state.

This book is about Roger Casement, the Irish-born member of the British Foreign office who investigated King Leopold II of Belgium's horrific actions in the Congo. Casement traveled about the Congo and interviewed people, which led to the release of the Casement Report. The Casement Report revealed Leopold had enslaved or killed millions of local people. His report led Leopold to give up the Congo as his personal domain. The Devil and Mr. Casement is about Casement's attempts to reveal the horrors Peruvian Julio César Arana committed in the Putumayo region of the Amazon against the local Indians, as Arana killed hundreds of thousands of locals in an attempt to extract rubber. The British Parliament investigated Arana's Peruvian Amazon Company, based in London, but Arana was left unpunished and continued to conduct abuses in the Amazon. 
One of the themes of The Devil and Mr. Casement is the difficulty of cracking down on abuses, even abuses as serious and extensive as those in the Putumayo region. While Casement arrives in the region with optimism that his investigation will end the abuses, "Casement's confidence soon dissolved into frustration and despair" (164). His attempts to arrest the perpetrators of the abuse in the region are unsuccessful because the political authorities in the region back Arana.
Another theme in this book is the flawed nature of justice. Arana, the perpetrator of great abuse, goes unpunished, while Casement, the humanitarian (who was also knighted), is soon afterward executed by Britain for helping Germany attempt to foment Irish independence from the British. The ways in which these two men are treated by the British justice system reveals the unfairness present in the British justice system at the time.