Marston was a reckless and irresponsible young man who had been convicted of killing two young children in a hit-and-run accident. He was a guest on the island, and his motive was revenge against Morris, the judge who had sentenced him. He poisoned Morris with the same poisoned chocolate that he himself would later die from.
The story is a complex web of murder and deceit, with each guest on the island secretly harboring a dark past. Marston's death is only the first in a series of killings that unravel throughout the novel.