Fundamentalism is dangerous. Period.
Doesn't matter if it has the banner of religion or politics or
any ideology. Science or Atheism could be equally bloody, if
they got into political post.
"We must be a pure [blank]" runs the risk of danger.
Find attempts at a pure anything that hasn't led to some
trouble.
Eugenics is recent for Science and Soviet Russian for a
corporate Atheism.
Argue as you may, extremism is dangerous wherever it comes from,
especially when it is mixed with political power - or the
opportunity for political power.