It's a thought experiment. It does not have to be hacking him up. The point is exactly to create a barrier that is sufficiently hard to cross to make the person think twice about making the choice.

An alternative might be to have the person carry the suitcase with orders never to surrender it, forcing you to be prepared to shoot him or order him shot if he can't be subdued, or any number of "easier" variants.

Part of the point is forcing people to think about why it is appropriate to order the murder of millions of innocent civilians if you are unwilling to kill a single person face to face.

There may be valid answers, such as if you have reasons to worry about further, more destructive additional strikes and want to try to take out their capabilities. But there are also many digusting reasons, such as being willing to carry out mass murder on innocents out of a desire for vengeance.

Eh, it was a propaganda attack on "Warmonger Reagan", who'd just become President, and the military he supported, disguised as a thought experiment.

There's no reason it can't be both.