The key part of the word "terrorism" is its root: terror.
Most crimes are committed for their immediate benefits to the criminal: A bank robber robs a bank so that he will have the money that he stole.
For a crime to be a terrorist crime, the point of the crime has to be to induce terror that goes beyond the immediate proximity of the crime. In addition, that has to be the intent of the crime.
Sure, a guy robbing a bank at gunpoint may cause terror among those in the bank; if that's all, then it's not a terrorist crime. It might even cause others, who were not in the bank, to feel terror at the idea of going to a bank, for fear that the same thing might happen to them. But it's still not a terrorist crime, because the purpose of the crime was not the terror; the purpose of a simple bank robbery is just loot.
For a criminal to be a true terrorist, he has to have an agenda, the furtherance of which (by way of fear induced in the public) is the goal of the crime.
The sticky wicket is that that places "terrorism" in the same category as "hate crime": thought-crime. And that way lies a legal morass.
The handy thing about terrorist crimes are that, in order to actually induce terror in the populace, you have to do something pretty heinous (like fly a plane into a building, or blow up a cafe full of people). Which means that we don't need "terrorism" as an aggravating element of a crime; you blew up a cafe, killing 20, and we have you on 20 counts of Murder 1. Congratulations, you've won free (but very very brief) medical care from the State.
Whether or not the act of flying a plane into the FBI/IRS building qualifies as a terrorist act depends entirely upon whether the man's intent was to invoke terror in the populace, rather than simply to kill a few people on his way off the mortal coil. And it doesn't matter. His act is neither more nor less reprehensible because of his motivations for committing it.
-BP
