Terrorists are not criminals in the sense that burglars, drunk drivers, perjurers, rapists and litterers are criminals. Terrorism may incidentally be a crime by its inclusion in the statutes that make it one, though pretty much any single action in the modern state is a crime by that standard.
The proper paradigm with which to approach an understanding of terrorism is not the criminal justice paradigm but the warfare paradigm. Terrorists are to be “prosecuted” on the battlefield (ie, wherever they are found) by soldiers, not prosecuted in the courtroom (ie, where they have to be brought and given “rights” as criminal defendants) by attorneys working for the state. That is to say, they are to be incapacitated and disarmed. If they fail to surrender, they are to be annihilated.
It may be proper to consider terrorists “war criminals.” But a war criminal is not essentially the same thing as a criminal. Adolf Eichmann is a good example of a “war criminal.” This guy was so ordinary and uninteresting that, thanks to Hannah Arendt, most educated people think of him immediately when they hear the word “banality.” Eichmann did not commit crimes outside the context of war and he was probably incapable of doing so. He did not have a criminal character and I can’t consider him a criminal. For this reason, I hate the expression “war criminal” and I consider it to be used primarily by people who think that war itself is a crime.
The question of how to treat terrorists, as enemy warriors who’ve engaged in atrocities or as criminal defendants with rights, goes deeper than just the question of warfare paradigm versus criminal justice paradigm. It comes all the way down to morality. If you have a Jewish morality, you hate evil and you want to see evildoers punished whenever and wherever they appear. If you have a Christian morality, you may hate evil, but you must love evildoers on some level, because their souls can always be saved.