Kill Flavor (or Modus Operandi) is a game mechanic in which flavor is given for how players who died overNight were killed.
Traditionally, Vigilantes and Mafia roles kill by shooting their targets ("shot", "killed with a bullet", etc.). Serial Killers usually kill by more blunt means ("knifed", "stabbed", "chainsawed", etc.). However, in nontraditional flavor, kills can be ascribed to just about any reason the moderator can imagine.
The key reason behind the importance of kill flavor is that it may reveal something about the killer. In games where kills are indistinguishable, the moderator will either use the same flavor for all kills or just make up whatever flavor they like each time someone dies. In games where kill flavors are identifiable (as in the traditional flavor), it is possible to identify which faction killed a given player, especially in games where the factions have different kill flavors (i.e. Mafia and Werewolves will shoot and maul their victims, respectively).
In a Normal game, all kill flavor must be the same, and should avoid giving any extra information. For most Normal games, kill flavor is along the lines of "died during the night", "was found dead this morning", or even simply just "died".
Individual Kill Flavors
In some themed games, kill flavors will be ascribed per player (or more precisely, per role). In other words, kill flavors will change based on who performs the kill, even between players on the same faction. This allows the Town to have some idea of whether the scum are sending the same player to make the kill on each Night. However, if the kill flavor is too specific it can lead to a flavor role being identified as the killer - not a good thing when those kills are obviously Mafia-driven and players' roles are confirmable. Thus, one must take care to ensure that there is some vagueness in the kill methods in order to prevent the game from being broken.
One mechanic-within-a-mechanic that has been tried for individual kill flavors it to only use the killer's individual kill flavor 60% of the time. The other 40% of the time, the kill flavor is some unrelated generic cause. This forces the Town to pick which kill flavors are real before trying to match them to a role.
In addition, in games with individual kill flavors it can become obvious which kills are made by the Serial Killer. Whereas Mafia players can send anyone on their faction to kill, SKs only have themselves. Thus, theirs will be the kill flavor that is the same every Night.