The deadliest active shooter incident by a single suspect was the recent murders in Norway, where one gunman shot dozens of students, trapped on an island with him, after setting off a bomb in the capital. To some, this has been classified as a terrorist act (Terror in Norway and Global Threats). Technically, that could be true, as he wanted to influence political views using violence on citizens.
The bottom line is that, regardless of motive, the episode was an active shooting. Once again, security professionals and law enforcement have to look at the incident and try to develop a way to prevent something similar from happening once again.