So is this guy a terrorist?
I believe terrorism needs a tight definition that is not broad ranging.
So from the Oxford Dictionary we have:
..."The unlawful use of violence and intimidation, especially against civilians, in the pursuit of political aims."...
Violence, or even intimidation, whether the perpetrator(s) are civilian or military, radically religious or not, is immaterial. Also, whether the target is civilian or military or religious, is not germane.
The last phrase is the most critical. The violence or intimidation -must be done with a political agenda in mind.
I see that as a person or group developing and executing a plan, however more or less detailed, because of a perceived political gain by act.
So that cuts out a LOT of responses to terrorism that are retaliation to terrorism based on blind reactionary hatred.
Thus, anyone in the mosque will do, not just the radicals actually planning a politically inspired act of violence.
I do not know if the perp in this case was acting on behalf of his politics- e.g. terrorism; or whether it was reactionary rage and retaliation, which is not, by definition, terrorism.
But make no mistake, unlike every other religion, islam IS political as the end result of a radical islamic takeover is changing United States', or any other nation's laws to conform to sharia.