Nah. Maybe an “insurgent” 
Revolutionary or resistance might be more correct.
He absolutely is a terrorist. There was no objective to his act other than to terrorize the IRS and make it think twice before ruining another man’s life.
Of course. He was a honest businessman who went crazy. However his actions were evil and were against the non-aggression principle. Contrary to what Ayn Rand said about “big businesses being the most persecuted minority”, small businessmen are one of the most persecuted and hated minorities in our society.
But was this about punishment? Maybe it was about self-defence. Could it be that rather than wanting to punish anyone he figured the only way to stop the continued aggression against him was to kill the aggressors?
You’re talking about it like he didn’t willingly commit suicide in the process. There is no debating the morality or justice of his act. Those things are for the living.
He stopped the aggression against him because you can’t aggress on a dead man. Anything he did up to then was just revenge.
Well he knew waging war on the IRS would be his death, but why did he enter into it? Was it really about revenge? Would he then had gone through with it even had they ceased their persecution so as to avenge past aggression? Or was it because he figured the only time it is ever going to stop is when one of them is a corpse?
being an aggressor does not make you a terrorist.