The custom described in 15:6–8 is not known outside of the Gospels, but all four Gospels mention it (Matthew 27:15–21; Luke 23:18–25; John 18:37–40). In many cases, ancient societies released prisoners during special holidays.
Barabbas: See also Matthew 27:16–17; Luke 23:19.
who had committed murder during the insurrection: Pilate's willingness to release Barabbas likely shows that Barabbas had not killed Roman citizens or soldiers (Matthew 15:15).