Gregory the Dialogist

Either that they had persecuted to the death, or that they had seen and were not changed. For the death of the saints is to the good an aid, to the bad a testimony; that thus the wicked may perish without excuse in that from which the elect take example and live.

Catena Aurea by Aquinas ↗

Hilary of Poitiers

And by this their testimony not only was all excuse of ignorance of His divinity taken away from their persecutors, but also to the Gentiles was opened the way of believing on Christ, who was thus devotedly preached by the voices of the confessors among the flames of persecution; and this is that He adds, and the Gentiles.

Catena Aurea by Aquinas ↗

Thomas Aquinas

And you will be dragged before governors and kings, as to Herod and many others. But you should have great consolation, because for my sake, whom you love. Augustine: "Love does all things as though they were slight or nothing." Also, "blessed are those who suffer persecution for justice' sake..." (Mt 5:10). And what will follow from this? This will be a testimony before them, i.e., against them, i.e., against the Jews and gentiles. For since they will deliver you up to the council, that will be a testimony against them. …

Commentary on Matthew ↗