What was Mr. Justice Wargrave accused of?

Justice Wargrave is one of the eight people invited to an island near Devon, England. Wargrave is a retired judge.
The voice on the recording played for the guests accuses each of them of murder. It names Wargrave in a crime during a specific trial, when he sentenced Edward Seton, who was probably innocent of murder, to death.
All of the accused guests insist on their innocence. Wargrave says he has a clear conscience and that Seton's sentence was correct because he had been convicted. Another of the accused guests, Dr. Armstrong, challenges this. He says that there was talk of the judge's prejudice against Seton, which might have led him to influence the jury toward finding the man guilty.
As the guests die off one by one, in manners resembling the ways their alleged victims died, Wargrave continues to insist on his proper conduct.
More complications ensue as the deaths mount up, but Christie finally provides the mystery's solution.