Worse, their reaction is an admission that their inhuman viciousness lacks empathy entirely.
Strikes the cognitive dissonance when they've been told what they're doing is meaningful, as such they can't come to grips with their robotic, Pavlovian actions.
The left have been programmed into mindless drooling puppets, and they hate being called that not because it's mean-spirited, but because it's the abject, demonstrable truth.