Trade-offs between decisions that secure immediate safety versus the "overall good" - whether that means a quicker end to war, more net lives saved, keeping the upper hand - are often made during wars. They're made in our everyday lives as well, though, of course, the consequences of choosing between those two options are on a much less significant scale. I thought reading about the deliberate withholding of the Zimmerman Note from Wilson was very interesting, as it was the first time I was exposed to this information. It didn't particularly incite a strong reaction from me, however, in the sense that I felt very pro- or against the decision Admiral Hall made.
At the same time, someone might argue that it was based on similar principles as those during the bombing of Nagasaki and Hiroshima, an event that I do have a strong opinion on. In order to end the war quickly, thus the "greater good" aspect of it, America decided to end 129,000–246,000+ innocent lives in Japan. While this death toll was not pre-estimated nor maliciously and intentionally predicted by any means, the result is still reality.
This differs from the Zimmerman event in that rather than deliberately taking lives, Hall put at risk the potential of danger from unrestricted U-boat warfare. I think the argument that his decision was reasonable can easily be made by stating that the reality of the situation was that the United States kept an upper hand and many lives were saved. It seems that the consequence was favorable. Taking that into mind, both these decisions - regarding the bombing and the Zimmerman note information - are probably considered ethical or reasonable almost overwhelmingly by looking at the result, rather than by looking at the initial intentions. If we looked at intentions, then both cases would un-controversially be deemed fair, which is clearly not true.
So by mulling over this event where we are in 2015, I too would most likely say (perhaps hypocritically, since I don't support the bombing decisions) that Hall made a logical decision to withhold the information he was keeping. Even knowing to consider his intentions, it is simply impossible not to look at the result when we're peering into the past from almost a hundred years into the future.