Sophocleese: The problem with your analogy is that there is "good cholesterol" as well as "bad cholesterol", and it is recommended that we ingest the good cholesterol for the benefit of our bodies. Cancer, on the other hand, doesn't seem to have any redeeming qualities, and only exists to destroy; it is recommended that we do all we can to eliminate it from our bodies by whatever means necessary before it kills us. Cut it out with a knife, bombard it with radiation, flood it with chemicals – whatever it takes to get rid of it – and be vigilant and ready to smite it should it appear again.

I don't think there's any such thing as a "good terrorist". Terrorism is indeed a societal cancer, and not anything which has any potential benefit to society, at any level of activity.