Well, Bush is seen as acceptable too. Or any US - or western - leader.
Downplaying the severity of despots like Mao by comparing them to democratically elected leaders is incredibly disrespectful to the 45,000,000 people that died as a direct result of catastrophic and coercive policies.
If you are so inclined, one was had good intentions, but backfired badly while other is explicitly cruel.
The only way Hitler could have gone out was in a pine box. That's the difference. He may have been democratically elected, but he wasn't a democratic leader.
Middle east is as it is currently largely to his fuckups and made up invasions for reasons barely better than russian invasion of Ukraine, and Afghanistan failure is proper second Vietnam for US to the last details, just less movies about it so far so its largely ignored and people act like it didn't happen.
Republicans still uncritically celebrate him, when I dared to criticize him even here I got downvoted to hell pretty quickly. Yet he is directly responsible for death of millions of innocent civilians and indirectly caused ie Isis, not on Mao or Stalin level but still.