Why do some theists bring up Adolf Hitler when discussing atheism?

[Minimalist]

As Seneca is believed to have said, "Religion is regarded by the common people as true, by the wise as false, and by rulers as useful."

It was not Seneca. It seems to be taken from a comment by Gibbon in Decline and Fall of the Roman Empire.

Chapter 2:
p. 25. The policy of the emperors and the senate, as far as it concerned religion, was happily seconded by the reflections of the enlightened, and by the habits of the superstitious, part of their subjects. The various modes of worship, which prevailed in the Roman world, were all considered by the people, as equally true; by the philosopher, as equally false; and by the magistrates, as equally useful.