Here are some thoughts on the following monograph, Monism: Science, Religion, and the History of a Worldview by Todd H. Weir.
Much of German socialism (and thus socialism generally) was apparently quite in bed with the likes of Ernst Haeckel (who provided a very popularized totalizing secular worldview that rejected religious belief). Not surprisingly this leads to many socialists who favored eugenics. It also tends to explain much of the arrogance and hubris of socialists in the 19th century.
The sort of all-encompassing efforts to explain all things also explains how Annie Besant can flip-flop from being an atheist to a theosophist yet remain a monist.
Monism also reminds me of the views of the new atheists in a number of ways (and why Gould was so skeptical of the approach of Dawkins, etc.). In particular the work of Sam Harris to create an objective secular ethics resembles the agenda of the monists.