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We can assume that those men who were imprisoned alongside Paul and Silas may have been murderers, rapists, thieves, etc. However, they too benefited from Paul and Silas blessing the name of God.
This action of freeing all who were imprisoned (regardless of the crime) shows God's mercy that is everlasting. His mercy is not dependent on our own personal merit either. According to human logic, those inmates did not "deserve" to be freed. However, God thought otherwise.
I also noticed that the verse said that the other prisoners were listening to Paul and Silas praising God. They were not complaining or yelling "shut up!". They were listening. Could this be one of the reasons God allowed them to be freed also? Just some food for thought.