There are two distinct meanings of the word terror. The first, and the earlier one in English (as I believe it also was in Latin), is a state of extreme fright; the second is the agent of such fright, like the Terror with a capital "T" that brought so much grief to French people under the first Revolution, or to Russians under Stalin.
Terrorists, as agents of terror, may not succeed in inducing it, any more than Flat Earthists can alter the shape of the world. But there still clings to terror a strong sense of the first meaning, of the paralytic fear that snakes can cause in rabbits: it carries at least a hint that the terrorists have already won, and that they really have brought their enemies to the state of inertia and dread implied by the original sense of the word.
Terror is one of those words that won't lie down, despite occasional attempts to anaesthetise it. The Gothick novel was supposed to bring terror to its readers, but of a pleasing sort, to tickle the imaginations of idle young women. In our own time it has been devalued often enough by the tabloid press, which uses it indiscriminately both for large-scale atrocities and for minor domestic upsets. ("Whirlpool terror of long-haired swim girl" - a headline in the Mirror about someone who caught her hair in an outflow pipe, bad enough at the time, but not in the same universe as the Lockerbie bombing.) Its other, second meaning has also been domesticated, but usually by fond parents, who like to use it of their own children. ("He's a regular little terror he is.") Yet it survives, a serious word.
The verb has suffered no such indignities: terrorise means what it says, someone who has been terrorised is in an undeniable state of terror. But a terrorist - whatever the immediate distress he has caused - may not, as I say, succeed. We should not flatter him into thinking otherwise.
What then? I tried the new Roget. Under the section "Violent creature", terrorist nestles between nihilist and revolutionist. Not nasty enough. Suggestions welcomed.