To get out of the weeds. If I'm not mistaken the question is about unconditional surrender and the ethics of such a thing. I don’t think it is ethical. Most wars (in my horribly amateur study of history ) are based on resources, i.e. land. Soldiers aren’t serial killers (usually). They want to go home and feed their families, despite all the politics.
Yes, you could argue that some enemies, well, can’t be reasoned with. (like here) To have a whole army of fanatics is rare. They are people, like your own soldiers.
So I’ll take it from the soldier’s level to that of the generals and the politicians.
Then things change. And we aren’t talking about military tactics, we are talking about politics.
“Unconditional Surrender” reeks of bad politics and poor leadership. You won.
If genocide isn’t your thing, then a heavy handed victory only brings resentment (and those wars that don’t seem to end).
To me the bottom line is….Your defeated enemy is either someone who could be a neighbor, or someone who should be killed.
It seems only politicians can remove that human element.