After Reverend Montgomery and Nigel de Grey deciphered the Zimmermann telegram, Admiral Sir William Hall refused to turn it over to the Americans. His decision not to tell President Wilson about its contents was not ethical, but it is somewhat understandable.
With the resumption of unrestricted submarine warfare, German U-boats were prepared to attack any and all ships in the war-zone waters, including civilian passenger carriers. Given the high number of lives at risk, it was definitely unethical for Admiral Hall to withhold this information. Hall knew that the U-boat onslaught was going to begin in a mere two weeks, and the fact that he did not notify the United States reveals his apparent insensitivity for the lives of others.
However, war is often about taking risks. It could be argued that keeping the Zimmermann message private was a beneficial risk to take. Preventing the Germans from finding out that their encryption method had been compromised may have actually saved a greater number of lives than disclosing the message would have. If the Germans had found out about the decipherment and had created a new and stronger encryption method, the British would no longer have been able to easily decrypt their messages. This could have led to a longer war with a higher death toll.
It is unknown whether Admiral Hall was simply being selfish in keeping the message from the Americans or if he truly thought that it was in everyone’s best interest to keep the message private until it could be disclosed without the Germans finding out that their cipher had been cracked. Either way, it was undoubtedly unethical, yet reasonable, of Admiral Hall to withhold the information from the Americans.