In Avot we learn that one should judge all people favorably ("והוי דן את כל האדם לכף זכות"). I would like to know if that includes the deceased. Meaning, can I look back on the life of a person who is now dead and, in reviewing his actions, judge them any which why I choose? Or, would the precept of judging favorably still apply?
Well, there's a חֵרֶם קַדְמוֹנִים to talk badly about the deceased.
As the Chafetz Chaim writes in הלכות אסורי לשון הרע - כלל ח
דְּיֵשׁ תַּקָּנָה וְחֵרֶם קַדְמוֹנִים, שֶׁלֹּא לְהוֹצִיא לַעַז וְשֵׁם רָע עַל הַמֵּתִים.
So if you wrongly accuse the deceased about something, you are the subject of an ancient excommunication.
That would be a strong impetus to judge them favorably.
Or, as the popular expression goes:
(Explanation: After they died they are Holy; a play on words of the 2 Parshiot Acharie-mot and Kedoshim.)
It would seem that according to the Rambam and others the principle of judging others favorably is an act of piety (middas chassidus) and not, strictly speaking a halacha, unless perhaps that person was a tzaddik. The Chofetz Chaim seems to disagree and holds this to be an obligation. In either case there is no indication that the status of the person you are judging changes whether or not he is alive. Furthermore there is every indication that this precept applies specifically to judges in a court case.