Exactly. They had been convicted, sentenced, and punished for a murder they didn't commit. If they committed the murder after the fact, they could not be punished twice for the same crime.
Someone can not be tried twice for the same crime. But what is the same crime?
A person can be robbed and survive many times.
So burgler A could be tried and convicted of robbing the house of house owner B on January 1, 1990, and on July 1, 1995, and on December 31, 1999, for example.
Highway robbery is defined as a crime.
Ten different men could be tried and conficted of robbing ten different travelers on the highway at different times. The crime has the same name "highway robbery" each time, but it is ten separate examples of the crime of highway robbery so it is not "The same crime"
A highwayman could rob ten different stages with ten different sets of passengers and could be arrested, tried, and convicted or acquited of ten separate crimes of hightway robbery in up to ten separate trials if there was one trial for each crime.
A serial kille rmight murder one person every month. Suppose that he is arrested for killing victim A on January 1, 2010 but is acqutted. Suppose that he is arrested again for killing victim B on May 13, 2010. Could his lawyers agrue that he can't be tried trwice and thus can't ever be tried again for murder, now matter how many murders he might get arrested for?
A person can survive many different attempts to murder them. And the separate or repeat offenders and/or conspiators in each attempt can be tried separately and convicted or acquitted separately for each act of attempted murder.
So if person A is tried and convicted and serves time for murdering person B on January 1, 2000 and is sentenced to only 10 years in prison, and person B turns up alive, and person A gets out of prison in 2010 and murders person B on December 31, 2010, he can be tried again for murdering person B, this time for the spearate crime of murdering person B on December 31, 2010 instead of on January 1, 2000.
That is my interpretation of the law.