I think the current behavior makes more sense. Kherab's creations cull the weak adventurers, while Kherab himself corrupts the strong with promises of power. It would be bizarre for him to get angry at you for filching the items he's using to lure you in.
Kherab: "I'd like you to kill the prominent foes of chaos. In exchange I'll give you powerful artifacts of dubious value."
Kherab: "What's that? You picked up that other powerful artifact of dubious value? What weak moral character! STOP THIEF!"
If you're looking to make darkforge worse, a more consistent way would be to make the other items more likely to be of dubious value: cursed, of vampirism, corrupting, chaotic, ashen, hateful, of damnation, of the void, vile, of anger, of hate, etc.