Why is the mere image of a zombie so universally repulsive? A theory in the field of robotics and computer animation called the Uncanny Valley may have the answer.
The Uncanny Valley holds that as artificial representations of humans become more realistic they are also more appealing, but only to a point. When a facsimile looks and acts too much like an actual human it repels viewers. Applied to zombies, the disconnect between our intellectual understanding that the the undead aren’t people anymore and our emotional urge to see them as somehow still human is at the root of our repulsion.
The graph below illustrates the sharp drop in positive human reaction in the Uncanny Valley, with zombies causing the strongest negative response.