@SauceEmperor I think it's because only the iris is drawn within the eye, without a pupil.
Often in manga/anime when there's no pupil drawn/shown it's either some form of stylistic choice showing a "knocked out" or unconscious state, or it's some sort of "hidden/eye power", eg the Byakugan from Naruto, or Kaguya with her weird all white eyes.
It might be because when we look at someone we tend to focus on the eyes, & not seeing a pupil weirds us out.
Human Brain: No pupil = something other, or not human. Therefore there must be something bad/evil about them.