What is the main theme throughout the novel?

The “mystery” referred to in the title is the disappearance of Edwin Drood. The plot revolves around the loves and jealousies of Edwin; his dear friend and one-time fiancée Rosa Bud; and a brother and sister, Helen and Neville Landless. Edwin’s uncle, Jack Jasper, also figures prominently in the story. Jasper became the young Edwin’s guardian after his father died and has since become an opium addict. There is a second “mystery,” however, which is how the novel turns out. It was Charles Dickens’s last novel, and he died before it was completed.
Neville has fallen in love with Rosa, whom everyone thought was still engaged to Edwin, and fought publicly with him. As he was the last person reported seen with Edwin, immediately he is suspected of having killed Edwin, even though no body is found. Jasper, however, has secretly been in love with Rosa. After she leaves their country town for London, he visits her there and he tries to blackmail her into a match with him in order to save Neville from being convicted of Edwin’s murder. Rosa meets with Helena and tells of her suspicions that Jasper has killed Edwin; she rents rooms so she can stay in London. A few apparently minor incidents are all that remain in the text Dickens left.
Because it seems that Jasper is experiencing a mental breakdown brought on by the opium—and may even have become a murderer—the evil that can lurk in the human heart is one major theme. The focus remains on the young lovers, however, so the primary theme seems to be the importance of true love and the related danger of jealousy.