I do believe there are lines that one shouldn't cross unless one wants to become the monster he despises.
I don't believe in engaging in a process that leaves permanent bodily damage. I don't think even in the hypothetical given I could chop off digits, or set someone on fire. I could and would kill in self defense of my family. But I don't think I'd be able to stomach any intensely brutal torturing process as a means of getting what I wanted.
That said I wouldn't have an issue with things that did not cause permanent bodily damage but did cause intense in the moment discomfort. Waterboarding, sleep deprivation, etc. I would probably also not have issue with punches in the face, smacks upside the head, shoving, etc. But could I take pliers and chop off another persons finger in order to extract info? No.