Workplace violence? Consider:
* Farook and Malik gathered weapons and ammo (he had owned the handguns for a couple of years, but borrowed the rifles from a friend);
* F & M obtained Kevlar vests;
* F & M obtained and learned to use an RC car and obtained explosives;
* F & M chose an event and venue which they knew was planed and Farook had attended in at least one previous year.
This shooting was carefully planned, over several months. It was "workplace violence" only in the sense that Farook slaughtered coworkers (whether any of the ones he killed knew him is an open question, though) rather than strangers with whom he had no association. I think "workplace terrorism" looks to be a more accurate description.