They killed in the name of socialism. Socialism is not atheism. One does not need a religious belief system to accord other persons with dignity and respect, including their rights to acquire and possess personal property from their own honest efforts.
I could go on and on with a list of people who killed in the names of various religious traditions, but you'd counter me with the notion that "those individuals were appropriating religion for their own selfish aims, and were not truly religious." I'll submit to you that Stalin and Mao were doing the same thing in putting atheism in their stated political philosophies. In fact, what they were doing was substituting their own cults of personality as the religion of the state. They didn't seek to destroy religion, they just wanted themselves set up as gods.
I think you are exactly right.