"Terrorist" is a much overused word today.
A murderer kills for pleasure, or personal profit. A terrorist kills to force some political or social change.
If there is no stated reason for the attack, no one claiming it was done "because" or this or that reason, even though the attack may generate terror, I don't see it valid to call it a terrorist act.
It is also possible that a valid "terrorist" may not be acting in concert with a group.
The fact that this guy was a convert to Isam makes some things much more likely, but doesn't alone make them fact.
Soldiers, Police, Politicians, Judges, civil servants, etc. are all representatives of government. in the eyes of most, attacking them is attacking the government, and attacks against government, either because of what it is, or what it is doing are today called "terrorist acts". Earlier generations called them "revolutionaries", or just crazies, wackos and nutjobs.
It is entirely possible that fellow went off to battle what he saw as "the Great Satan" with a stolen deer rifle (and apparently no extra ammo), never intending to come back. We'll never know for certain unless we find something he left behind saying so.
Unless something like that turns up, speculation, while entertaining, is essentially pointless.