From SA Mafia Wiki
A Death Miller is a Miller who, upon death, flips scum-aligned, even though they actually win with the Town. The Town will likely spend all game thinking that player was scum, and won't know until the setup is posted at the end of the game that the player was actually a Townie.
Death Miller is seen as unbalancing, because cardflips are considered confirmed information, and many players view them being altered without a way to discover the truth as unfair. Through confirmation bias Death Millers mess with players trying to make connections between other players, creating the risk of false positives among innocent townies. It is likewise seen as unfair for the Death Miller, because there is typically no way for the Town to know the Death Miller's real alignment, meaning the Death Miller's posts and cases will be seen in a scummy light all game.
There also exists the super-rare Reverse Death Millers, players who are scum-aligned, but upon death flip town.