I’m not sure why people are struggling with this one. The reason Hitler is at the top of the evil ruler foodchain is because of his ideology and specific intent.
He wasn’t some common racist who thought his own race was superior and others were inferior. He specifically believed that Jews (as well as Roma people, gays and lesbians, people with disabilities and mental illnesses) were vermin. Worse than insects. He openly made it his goal the complete annihilation of all these peoples who he hated with all the virulent, vitriolic, frothing-at-the-mouth passion he could possibly muster. He engineered the industrial killing of over 11 million people.
There is no other person more deserving of the label “enemy of humanity” than Hitler. There is absolutely nothing for which any decent person can relate to him, never mind understand him.