I just had a random thought just now. At least three times this story arc Zoro has been seen with a rather evil expression on his face.
First when the drawbridge was lowering (as pointed out by Nami)
When he first confronted Kaku ("It just won't stop howling about how much it needs blood.")
! Upon being freed from Soge King ("You missed your only chance to kill me easily, World Government!")
In the second and third instance, Zoro had out, or was in the process of drawing, Sandai Kitetsu. Also, he displayed an unusual connection with the Kitetsu sword, knowing Chopper had it when he was head-first in a chimney.
This made me wonder, is it at all possible that Sandai Kitetsu's supposed curse is beginning to have an effect on Zoro? Possessing or controlling his actions in some (currently) small way? If so, is this something that will come up now, or later on?