It seems obvious that one shouldn't speak badly or in a negative way against anyone whether they are living or not. However does it say anywhere that particularly one should be careful about speaking negatively about a deceased person?
The Shulchan Aruch 606:3 writes
ג. תקנת קדמונינו וחרם שלא להוציא שם רע על המתים:
There is a takana (enactment ) and a ban that one shouldn't speak evil about the deceased.
I think it is a common misconception that speaking evil of the dead is worse than the living,but as one can see it is not true. Speaking ill of the living is worse.
The Chofetz Chaim is explicit that it is forbidden.