A genuine historian absolutely must not pronounce moral judgement (this is the job for priests and secular ethics professionals). It does not mean, however, that the historian must not judge (evaluate) Nazis at all. He or she must, of course. However, not Nazis as human beings (only the Almighty God has the right to do that), but their words, choices, decisions and actions.
And there is one and only one scientific procedure to do that. First, one must understand in sufficient detail their situation and environment – their key problems, threats, opportunities, limitations, available resources, etc.
Second, one must identify and evaluate options (choices) available to them in that situation at that time and in that environment. In other words, a comprehensive set of alternatives (decisions and actions available to them).
In addition, one must identify the comprehensive (necessary and sufficient) list of criteria (quantitative and qualitative) that one will subsequently use to evaluate actual decisions made and actions undertaken by the Nazis. Criteria relevant to their times, environment and situations – not to our times, of course.
Third, one must identify the best possible alternative (according to abovementioned criteria, of course) and compare this best alternative to actual decisions made and actions undertaken by Nazis in question.
That’s the only scientific (i.e. correct) way to do it. Anything and everything else is not science, not history but propaganda (either pro-Nazi or anti-Nazi). Politics (and management in general) is the art and science (sometimes more of the former, sometimes – of the latter) of making the best possible decision in a specific situation. Period.
Thus the only genuinely scientific way to evaluate and judge the Nazis (from Adolf Hitler to the lowliest SS-man) is whether they made the best possible decision and/or undertook the best possible actions in their situation and environment. Nothing less, nothing more than that.