Jeff - I'm not sure you're correct in saying that Iain Banks is of the opinion that torture is a legitimate course of action in certain (special) circumstances. The Culture (the author's galaxy-spanning human/AI civilisation for the uninitiated) isn't always painted as the hero.
It frequently commits unspeakable acts (as in Look to Windward) in the name of 'the greater good'. I would suggest that what Banks is saying is that evil is still evil, regardless of it's justification or the sorrow and regret felt afterwards. The Culture has always been an ambiguous entity - a bunch of nice, intelligent, liberal folk with immense and terrible power.