A Serial Killer is a scum player who acts essentially as a single-person Mafia; their win condition is to be the last player left alive at the end of the game. Typically they are given a nightkill and also have some other added powers, especially in larger games. These can range from minor (a one-shot bulletproof, for instance) to extreme (Tom Tucker's Sylar from Heroes Mafia, a tracker-proof roleblock-proof doc protection-busting bulletproof-busting godfather).
Serial Killer Victories
For a variety of reasons, Serial Killer victories are very uncommon. Some argue it is because Serial Killers are so underpowered compared to a town and mafia full of roles and abilities; others believe it has more to due with pure luck (the larger the game, the less likely it is any one particular player will survive to endgame). But regardless, whenever an SK wins, we in the SA Mafia community celebrate. If you are one of the noble few who have won such a solo victory, you are part of an elite club. Pat yourselves on the back; you're probably pretty awesome.