Though everyone is raging about his belief in eugenics and saying that Hitler was also a proponent of said belief, the main difference is that Bell's argument was for more humanitarian purposes, if you will. He reasoned that if people with a hereditary history of deafness didn't pass on those traits, we would eventually be able to breed congenital deafness out of society. He likely viewed deafness as more or less a kind of suffering, since a large amount of his life revolved around the deaf.

On the other hand, Hitler's motivation was not a concern for people with disabilities, more of a racial-hatred-motivated one.