William Burkitt on Acts 12:18

Observe here, 1. How wicked persecutors are mightily enraged, when their bloody purposes are disappointed. Herod having lost his prize, is so incensed, that he caused the keepers first to be examined, (and possibly by tortures) and then to be put to death.

Observe, 2. The justice of God, and the great injustice of Herod: It was just in God to suffer the soldiers, who were Herod's instruments in persecution, to die by the bloody hand of Herod, whose tools they were. Instruments in persecution God oft-times meets with in this world, and sometimes they fall by the hands of persecutors themselves; yet was it notorious injustice in Herod to put to death those innocent soldiers, who could not help what was done, and were not ways accessary to the prisoner's escape.