He Wasn't As Bad As His Brother: Why Not Speak Ill of the Dead?

When Pinochet died, the title of JayinBmore's e post over at Are You Effin' Kiddin' Me was titled "Speaking Ill of the Dead." Ever since, I've been trying to figure out what is the basis for our prohibition against speaking ill of the dead. If we are told not to tell the truth about something, surely, there is good reason. Seems like it must be one of three possible sources:

A moral prohibition:
Is it moral in that it violates a sense of fairness to say something bad about someone who cannot defend himself? You can't properly judge without full understanding of the context and the person cannot speak for himself to fill in the missing details. Is it that the person has just suffered and you don't kick someone while he is down?

An etiquette-based prohibition:
Is it a matter of politeness, making sure that one does not offend the beloved of the deceased who are already dealing with the grief of their loss? Or is it that one's reputation lives on and you may be affecting how he is seen in posterity which also affects the family?

A religiously-based prohibition:
Or is it theological -- the person is having eternal judgment passed on their soul and you don't want to jinx them since so much is at stake or when someone dies it is God's place and not that of a mortal to judge?

What is so special about death that it overrides truth? Thoughts?