I'm not condoning his actions, far from it - but if we have nothing to hide, why are the number of civilian deaths considered a military secret?
The fact they are means he has, if guilty, done wrong and should be severley punished, but I don't understand why these figures should not be in the public domain in the first place.
Security classifications are often in place to protect the source of the information rather than to hide the information itself. If the information has come from a Humint source, then its publication could lead to exposure of the source, or at least suspicion falling on the source.
Another possibility arises when the information comes as part of an intelligence sharing agreement with an ally. There is then a protocol by which the information is never shared with anybody else - ie the information remains under control of the ally who provided it. This may have been the case here.