John Claggart is the one who accused Billy of mutiny and sets in motion the events that lead to his execution. Why he does this is complex, but the narrator often mentions both Claggart's essential evil nature, and his envy of Billy.

John Claggart is the one who accused Billy of mutiny and sets in motion the events that lead to his execution. Why he does this is complex, but the narrator often mentions both Claggart's essential evil nature, and his envy of Billy.