The mortal he has in his grip doesn't react quite the way they usually do. This startles Zack enough to give him pause, and perhaps not dry the mortal out from the inside, or have him eaten by rats. (He has no idea as yet, that his powers are useless here.)

Handsy? It takes him a moment to work that one out. Zack is pansexual, not that he'd dirty himself on a mortal. So if he is offended it's not for the usual reasons... only the human goes on to speak clearly and without fear. It might not answer his question, but it's good enough. Mortals frequently try to bargain their way out of certain death, even when their planet is crumbling around them. This one seems no different, but at least he's not cowardly about it.

Zack growls, but he releases the human. “Don't think you can cheat me. Nobody can.”

He might sound a bit more grateful.