There is nothing in secular humanism that is exclusively concordant with or solely pertinent to the atrocities of fascism or communist despots. Secular humanism ideologically rejects supernaturalism and any associated dogmas or superstitions in matters of ethics, government, or society. Instead, it proclaims that any institutionalized belief must be methodologically scrutinized, without taking anything on faith or justifying anything with doctrine. It also professes that the best way of achieving an ideal society is through free exchange of thought, the understanding of opposing viewpoints, and the of those who are superficially different from ourselves.
Mao Zedong, as far as we can tell, was not religious himself in any demonstrable way, and was oppressively antitheistic. But his regime is in no way one built on secular humanism. What he and his regime DID promote among the Chinese people was servility, mindless credulity, personality cult, indoctrination, suppression of free thought, violent social conformity, and the absolute suppression of ideological dissention. Not only are these things all at odds with secular humanism, they are also often the hallmarks of organized religion and theocracy.
As for Hitler, he goes directly against secular humanism entirely. He was a spiritualistic theist, who's religious beliefs were based both on Germanic Paganism and Christianity. He was a creationist, a denier of evolution, and believed that the extermination of the Jews was in accordance with the will of "The Creator." His belief in the Master Race and Nordic-Bloodline myths, as well as his pseudoscientific approach to racialism, are also diametrically opposed to the ideologies of secular humanism.
Pol Pot wasn't areligious either; he didn't believe in a god exactly, but seemed to be a spiritualist nonetheless, and believed he was divinely inspired and that his actions were part of a manifest destiny. His regime also catered to the doctrine of Buddhist nationalists, which he used to justify his unorthodox approach to Communism. So again, nothing akin to secular humanism.