As noted above, the permissibility of speaking negatively for a constructive purpose is based upon the principle that it is not lowly to relate negative information for a positive reason.
This applies only to information which is derogatory but harmless. However, it is forbidden to relate harmful information for a constructive purpose. The only exception to this rule is where one has determined that halachah allows for the subject to be harmed in that manner.
Sefer Chofetz Chaim, 19 Cheshvan, page 146