In what sense is being put to death a punishment? How we can talk about things like "suffering" or "loss" if a person is dead (i.e., not conscious)?

Most of Thomas' response focuses on your observation that once one's dead one's "not conscious", and he nicely tries to clear a space for the possibility of harm's being done to someone even if that person doesn't feel the harm. But in most of the cases he considers, there is still someone to be the subject of the misfortune: the clueless entrepreneur, for instance, is still around to have his interests set back (even if he's not aware that that is happening). Death is rather peculiar, however, in that it's a misfortune that eliminates from the world the subject of the misfortune. (Of course, someone's death might be a misfortune for others. But as you note, we put people to death to punish the very people who, if the punishment is carried out, are no longer around.) Once one's dead, not only does one cease to experience things but one ceases to have interests too. That's what makes your question hard. It's really the question the Ancients (and everyone else) argued about: whether one's own death...