I have always understood the Nazis because I am of that sort by nature.
Fanaticism in religion is the alliance of the passions she condemns with the dogmas she professes.
All empty souls tend toward extreme opinions.
There are aspects of Westboro that are, of course, more extreme in the way that certain religious practices manifest. But the idea that the Bible is the infallible word of God, that it's unquestionable - this is common.
When like-minded people deliberate as an organized group, the general opinion shifts towards extreme versions of their common beliefs.
Fanaticism is ... overcompensation for doubt.
There are a lot of Christian fundamentalists; there are a lot of Muslim extremists. Every religion - Mormonism - has something way on the side that's completely using the religion as some weird backbone for their twisted faith. It has nothing to do with their religion.
Very few minds are strictly normal, and all religious fanatics are marked with abnormalities of various sorts.