Bad actors can still make rational tactical decisions

How should historians combine ethical judgment of leaders such as Dönitz with analysis of their military decisions?

Condemn their conduct where warranted, but still reconstruct tactical choices from the information and capabilities they actually possessed.

Ethical relevance belongs in history, and figures such as Karl Dönitz can be morally condemned. That condemnation does not mean a bad person must make uniformly bad professional decisions. Scharnhorst's actions at the Battle of the North Cape provide the example. German commanders operated with incomplete information, underestimated Allied firepower, and had lost radar without an equivalent means of detecting Allied emissions. The resulting decision failed, but the intelligence available explains why it could still appear reasonable at the time.