They even include Hitler's Nazis as atheists, despite Hitler being raised a Catholic, singing in the church choir & contemplating joining the priesthood.
Whilst most of the massacres were carried out by atheists, the interesting thing is that all of them had quite religious upbringings.
Stalin studied in a seminary & was only kicked out because he couldn't pay his tuition fees.
Kim Il Sung was raised in a Presbyterian family, his maternal grandfather was a Protestant minister, his parents were active in the religious community & his father went to missionary school & was an elder in the Presbyterian Church.
Pol Pot attended a Catholic school.
Fidel Castro was baptized Catholic & attended 3 Catholic schools.
The exceptions who didn't have a christian upbringing are Mao who was raised a Buddhist & later studied Confucianism & Ho Chi Minh who was also raised in Confucianism & his father was a Confucian scholar & teacher.
None of these mass murderers were raised non-religious atheists.
History has repeatedly shown us what happens when a religion, whether it is theist or atheist, is universally imposed on a society therefore it is a very fine thing that we are losing our religiosity.
We just have to be vigilant that it isn’t replaced by another form of theist or atheist religion.