So I suppose the question is, for what value of harm does (harm per terrorist * small number of terrorists) become so large that very harsh interventions affecting large numbers of innocents start to seem actually reasonable? (Even considering that actual terrorists are not quite like the media's image of them, I think we should probably be willing to pay a rather large cost to keep x-risk-level weapons out of the hands of the terrorists we have today, so clearly there is some line...)