'Don't speak ill of the dead"

Yes, the earliest known use of this expression is in The Lives and Opinions of Eminent Philosophers written by Digenese Laërtius around 300 AD. In this he attributes Chilon of Sparta as saying "don't badmouth a dead man." Chilon was one of the Seven Sages of Greece - a title given by Ancient Greek tradition to seven 6th century BC philosophers and statesmen who were revered for their wisdom.

It probably made its way into modern day venacular via Abrogio Traverssari's Latin t***slation of Diogenes's book in the 15th century. It appears to have made its way to America with the early settlers as it appears in "Will and Doom." Despite sounding like the name of a rather violent computer game, "Will and Doom or the Miseries of Conneticut by and under an Usurped and Arbitrary Power" is a book by Gersham Bulkeley written in 1692.

While it is socially inappropriate in most circles to speak ill of a person in the immediate aftermath of their death, fortunately this belief does not hold sway for the long term or otherwise, to take an extreme example, we would have no recorded history at all. Putting aside public figures, while modern-day psychologists would likely agree that there comes a point when it is best to put something behind us rather than to continue to dwell on it, they would also likely agree that it is important not to white-wash over past events as the actions of the dead may not be able to physically harm us anymore but certainly can continue to do so mentally.