It was actually very consistent with Zoro's personality after the time skip. He trained with Mihawk who is known to leave opponents alone if the difference in power is so great. Then when they try something sneaky he demolIshes them in a single strike.
Think back to Fishman Island, Zoro called the drunken swordsman, Hyouzo, a frog in a well, same thing Mihawk called Zoro way back when. Zoro realized his opponent was nothing in comparison and walks away because fighting him wasn't worth his time, then Hyouzo tries the sneak attack and Zoro destroys him.
Monet is fighting Zoro and Zoro quickly realizes she is no where near his ability so he gives her the chance to walk away. She doesn't so he puts her in her place.
Never thought about it that way, but it is a break with how he was portrayed before, not that he went out of his way to torment foes weaker than him, but he certainly seemed to be having fun trashing fools on Whiskey Peak.