In Assassins' Guild games, when a large group of people are slain in or around the common room. Common room massacres can have several motivations:

Wrong Place, Wrong Time
Someone wandered across a crime or suspicious circumstance, which gets them killed. Another person wanders through the murder zone, and must be killed to prevent them from being witness. This can escalate into 20 or 30 kills quickly.
Free Fire Zone
Since many people feel more comfortable in a common room, some characters will choose this location as an assassination location, unmindful of casualties among the innocent bystanders.
Item Hunt
When all else seems hopeless (especially for those in an epic-level plot such as blowing up the world), characters may choose to slaughter a decent size group of people to attempt to gather items or information useful to their plot.