Because a much larger percent of dem- or at least democratically motivated within the vox-pop possess Consciences thus also too the capacity to Feel Shame (when acting shamefully.) Both qualities relate also to empathy: a word unknown to the National Hitler-wannabe. Thus his entire mentation is FUBAR, in all matters.

ie when All Three qualities are missing.. we can begin to comprehend the actual _{Depth of depravity} which IS this Monster's entire non-being
(he doesn't even rate a 'B' on that word.)

tl;dr: he's un-Believable because his Being-overall is murderousl-defective. Zombie-grade?