While it's tempting to blame poor ol' Nietzsche for all this, WW1 may have played a teensy role (the catastrophic consequences of the Treaty of Versailles and all).
It's not clear Nietzsche's atheism was a function of Darwin's theory. I can't think of a single passage where he states this. It seems he just asssumed it to be true (perhaps thinking, correctly, that after David Hume the burden of proof was squarely on the theist's shoulders).
It's worth pointing out that not all atheists are "militant," pro-choice, or liberal. More than a few simply shun metaphysical speculations on the grounds that the human mind has no business in that realm.
I've always found the militant atheist a baffling phenomenon: Why do they care? Especially in a country where there's no religious persecution or fanaticism?
The term militant was specifically employed in order to differentiate between the two sorts of atheists.
When I was an atheist I remember thinking I wish I could be so gullible as to believe in the fairy tale of God. It seemed like a pretty, pink fairy tale that gave it's believers some comfort that I lacked and wished I could be naive enough to believe. I envied them their opiate rather than hating them/and or their belief in their supposed God. After all what did it really matter anyway was the way I felt. That is why I think many atheist are actually God haters rather than unbelievers. If you truly do not think God exists what is the point of worrying about those who do?