Well torturing a terrorist is right punishment for this bastards, kinda. Remember this guys kills hundreds or thousands of people so they deserve it. But it is also morally wrong and y'know why.
I highly suggest a quick death is better for them rather than killing them slowly. Maybe hang them, electricute them, give them a firing squad, lethal injection, or just chop their head off. Basically this kind of "people" doesn't deserve any mercy at all.