From SA Mafia Wiki
Some common scum roles include:
Debate over Definition
While the definition presented above is the most common definition of this term, there is still a wide debate about what "scum" means precisely, especially in relation to third parties. Are survivors scum? How about jesters? And so forth.
Ecco's definition of scum: anyone whose victory condition either excludes the town from winning, or in some significant way debilitates them. For instance, the mafia win condition and the town win condition are mutually exclusive; mafia are scum. Conversely the survivor win condition is not mutually exclusive with the town win condition; they are not scum. But Jesters, in order to win, cause the town to waste their biggest tool against the mafia -- the public execution. Ecco considers them scum.
Scum is also frequently used as an adjective to describe players who are suspicious, as well as to express the opinion that a player is likely scum (e.g. "Windshipper is scummy for switching his vote so much," "Xythar is the scummiest player in the thread," "TLB is a Scummy McScumerson")