I understand your perspective.
I think it would be helpful to review the
Catechism’s teaching on the three elements by which the morality of an act is determined.
(Italicized below is Catechism, bolded mine)
- the object chosen
- the end in view or the intention
- the circumstances of the action
All three of these things must be GOOD, or else the whole act is evil.
*
The object chosen is a good toward which the will deliberately directs itself. It is the matter of a human act. The object chosen morally specifies the act of the will, insofar as reason recognizes and judges it to be or not to be in conformity with the true good. Objective norms of morality express the rational order of good and evil, attested to by conscience.
The end is the first goal of the intention and indicates the purpose pursued in the action. The intention is a movement of the will toward the end: it is concerned with the goal of the activity.
The circumstances, including the consequences, are secondary elements of a moral act. They contribute to increasing or diminishing the moral goodness or evil of human acts.
Circumstances of themselves cannot change the moral quality of acts themselves*; they can make neither good nor right an action that is in itself evil.
So, let’s look at the example.
The object is the killing of the man.
The end is the saving of the city
The circumstances are the details that the terrorist is forcing this situation, etc.
In this case, the object cannot be justified, because one wills the death of an innocent man, however good the intention may be.
It seems to me that you have said that the killing of the man would be justified because of the circumstances, but the Catechism is very clear that the circumstances cannot change the moral quality of acts themselves.
I know it’s hard, but it’s very clear that all three elements of the act must be good – and one element of this act is evil.