Good question.

The dead man, whomever is in question, can no longer harm you. He was a man, maybe a husband and father, and speaking ill of them is of no tangible benefit. To those that respected or loved them, the relationship is gone, and it is not wise to add to their pain.

I have been to the funeral of bad men. His earthly power is gone and if there is an afterlife his judgment is sealed.

This goes for all enemies and tyrants and criminals. We use the term "I am sorry for your loss" because most times the loss is not ours.

I suppose you shouldn't jeer at them for being dead, for a start, and you should make allowances for their being dead when judging their actions. Treat them fairly.

It's mostly because the dead cannot defend themselves. You are attacking someone who you have no fear of reprisal from.