I've seen several variations of this argument recently:
It would seem that every act of torture cannot be evil in its object. For every act of execution is not evil in its object. But execution is a greater punishment than torture. And since life itself is fundamental to all aspects of personal dignity, torture cannot be more contrary to personal dignity than execution. Therefore, all torture cannot be evil in its object.
To respond to this, I will first point out that the Catechism deals with both execution and torture in the section on the Fifth Commandment. But execution is mentioned in the subsection "Respect for Life," while torture is covered in the subsection "Respect for the Dignity of Persons." So, while life itself may be fundamental to all aspects of personal dignity, issues of personal dignity are not wholly subsumed by issues of life. In other words, a thing may be consistent with respect for life yet contrary to respect for the dignity of persons.
The problem with the argument is that it presents execution as the upper limit of a continuum of punishment. But that's not actually the case. Once you execute someone, he's dead. With patience and care, though, there's no fixed limit to how much torment you can inflict upon him.
And I'm not saying that torture can surpass execution on the continuum of punishment. I'm saying that there is no continuum of punishment, because a continuum assumes a single dimension and punishment has at least the two dimensions of life and dignity.
So the fact that execution need not be contrary to the personal dignity of the one executed does not imply that torture need not be contrary to the personal dignity of the one tortured.