I think it depends on the back story. Why is he skilled at torture? Was he raised by perhaps an assasins guild or evil person and forced into the skill reluctantly? If so he could easily be good and have that knowledge and background (and ability to do so) while be conflicted about it. I think the character in this case would be struggling with their own alignment. What he was raised to do dueling and feels like he has to, to protect himself/those he loves, with his beliefs or viewpoint in life.
However if you make him a good alignment and have him be skilled at torture just because he thought that was a cool trade to learn... that doesn’t make sense to me.