The shooting at an Aurora, Colo., movie theater screening of "The Dark Knight Rises" Friday underscored a chilling trend: While gun violence has plummeted over the decades, mass murder has increased slightly.Single-victim gun killings have dropped more than 40 percent since 1980, according to 2010 FBI crime data. But the total number of people dying in attacks that claimed four or more victims has climbed from an average of 161 a year in the 1980s to 163 between 2006 and 2008, according to FBI statistics.Shooting massacres do not mirror the more volatile statistical curve of individual gun violence.