Admiral Hall of Room 40 - Britain's analog to the American Black Chamber - was faced with an impossible choice during World War I: immediately release information of the Zimmermann Note to the Americans and risk the Germans developing new, more secure ciphers, or holding on to the note until the perfect moment, potentially risking thousands of innocent Americans. While Hall's final decision was morally duplicitous at best, it was certainly the more ethical of the two options, and one that, on net, saved more lives and brought about the end of World War I.
The choice to release the Zimmermann Note to the Americans was one of the most pivotal decisions made during the Great War. To fully comprehend the implications of Hall's decision, we must analyze the logical end of both options he was presented with. First, we'll consider the case where Hall releases the Note immediately. As history has proven, upon receiving the Zimmermann Note, American politicians almost unanimously motioned to go to war, Woodrow Wilson even reneging on his campaign promises and urging Congress to approve an official declaration of war. Of course, such a momentous decision would be heard the world over, documented by every major news source on the planet, especially in Germany; these stories would likely also detail the reason why the United States changed its mind: the Zimmermann Note. Following this, it would not take the German government very long to deduce that their encryption techniques had been broken, forcing them to engineer new ways to encipher their confidential information. By forcing German intelligence to upgrade in such a way, Great Britain would have been swamped with a sudden influx of cipher text that needed to be decoded, cipher texts that would demand resources to decipher. In this way, the British would be stuck playing intelligence catch-up, as the Germans would thus be able to their troops around freely without the Allies knowing. This would clearly lead to a colossal loss of life on the Allied side.
On the other hand, history has shown that Hall's strategy ultimately paid off, and that the number of civilian ships sunk by the German's aggressive UBoat campaign were few and far between. Therefore, in true utilitarian fashion, history will and must regard the choices of Admiral Hall as ethical insofar as they mitigated an excessive loss of life and expedited the end of the war.