Designating enemies as "allies"

[Kordeth]

Then the blow should have been lethal.
That is, after all, the definition of lethal .. I mean, if you knock the wrong target unconscious, you can still kill them later.
Choosing to deal lethal should mean just that.

Except that it's entirely the DM's perogative to decide which enemies are assumed dead at 0 hp and which go to negatives like PCs, so there's nothing stopping Colmarr's DM from deciding that, by virtue of the PCs wanting to interrogate him, this particular enemy was important enough to warrant such added survivability.