What did general MacArthur accuse arthur Richmond of doing in and then there were none?

In Agatha Christie's "And Then There Were None," General MacArthur accuses Arthur Richmond of having an affair with his wife, which he believes led to Richmond's death during World War I. MacArthur harbors feelings of betrayal and guilt, believing he sent Richmond to his death as a result of this betrayal. This accusation reflects the themes of guilt and justice that permeate the novel.