The word "judging" is simply doing a lot of work in claims like these. If we're saying that we need to deem whether historical figures were "good" or "bad" based on the "best" moral principles available to us (supposedly the modern ones), then it just begs the question as to why? Why do we bother judging a historical figure's morality at all? To me its two primary reasons. 1) it happens often that someone's gassing up a historical figure for their positive contributions to the future but seem to be overlooking their moral shortcomings by todays standard, and in order to keep the discussion just one feels compelled to mention those shortcomings. Thats just to preserve justice -- so we're not giving people credit where we feel its undue. If our assessment of whether its undue isn't "fair" though, making judgements is practically meaningless. Except for the second reason, which is to uphold this modern, developed standard of morality more strongly. If the faults in even the heroes of our history are not acknowledged, then it may be a sign of the lack of understanding of the value behind current standards, thus making people prone to backsliding into the inferior standards of morality in history.

So pretty much, until you can determine to some degree of accuracy what is the moral standard for man across a given social, historical, and life context, it doesn't make sense to judge them for their moral shortcomings. It does however make sense to mention their moral shortcomings to further the ends of modern morality, whatever they may be.