Answer: Hall knew that growing outrage among the American public would help Britain's cause.
Explanation: According the the excerpt, it says that "Hall reminded them", thus saying that Hall was not in favor of waiting before presenting the telegram.
Hall didn't want to wait until American outrage simmered down before revealing the contents of the telegram. No where in the excerpt does it hint this.
Hall wasn't worried that the Americans would lose interest in the contents of the telegram. The telegram contained information that would alert the Americans of a possible attack from Mexico.
Hall did disagree with the waiting, but the excerpt is hinting that Hall wanted to use the telegram to gain supporters for Britain against the Germans.
Cheers
(If I'm wrong, then the answer is that Hall disagreed with the British government's choice to wait before showing the telegraph to Wilson)
