But if we're concerned about identifying the root causes of this problem (whether it's religious extremism or workplace violence) to reduce future incidents, then we also need to investigate all of the contextual factors that led to this incident.
Agree, and it is seldom one thing that leads to a behavioral change. This is my concern with what I'm calling deterministic models. They attempt to isolate a specific variable, and then context is lost and the truth is in the larger context. There is also a good chance that the truth can't be determined.

And since you mentioned idealism and ideology; the general consensus in academia is that ideology emerges from social practices and human activity not vice versa. So we can grant that an individual may act on idealism, but to understand the specific ideology, its characteristics, and its appeal, it's also important to explore its relationship to the material world. Why radical Islam and not radical Christianity? Why beheading and not shooting? Why target the workplace and not neighbors? Why did the suspect feel compelled to promote conversion at his workplace in the first instance? People do not behave in a vacuum and they are constantly responding to and assessing their material conditions.
I'm not yet convinced by academia's chicken or egg claim. Ideology is a worldview, and regardless of the character of the material world the will perceive it and act upon it through that lens. Is their an interdependence between the material world and ideology? Most certainly, and each shapes the other, but to claim ones must first doesn't seem provable to me.