As consul he had been defeated in Africa and taken prisoner. Years later the Carthaginians sent him to Rome to negotiate an exchange of prisoners, on one condition: if it failed, he would return. He gave his word. In the Senate he was granted the floor and advised his own city against the exchange; the swap favoured the enemy, he said, and he himself was old and not worth it. The Senate followed his advice. His friends pressed him to stay; nobody would reproach him. He left for Carthage because he had said he would.
Marcus Atilius Regulus was defeated near Tunis in 255 BC by an army led by the Spartan mercenary commander Xanthippus, during the First Punic War. The story of his return comes mainly through Cicero, who uses it as the textbook case of fides, keeping your word even when it harms you. The gruesome details of his later treatment come from late and literary sources; Polybius, closest to the events, does not mention them, and modern historians treat them as embellishment. What remains is the core already in Cicero: he went back because he had promised.
Everyone would understand if you stayed. No court will convict you. Only you know what you said.
The war, the outcome of the vote and what Carthage would do with him: all outside him. His advice to the Senate and his return were his, and precisely because nobody could compel him it was a choice and not a duty. Note that he advised against his own interest: he personally made sure there would be no exchange that would have saved him.
Which agreement do you keep only because you made it, even when nobody would notice? And which one do you not?
Yours, Not Yours · A Stoic practice ground · All cases · Questions or suggestions