Violent religious fanatics are a real
danger, but
there are no violent atheist fanatics nowadays. The closest thing
you can find are a few Communist dictatorships. They are cited to
create a false appearance of moral equivalence between secularism and
religion.
People do not need religion in order to treat each other well. Humans
(except sociopaths) have a conscience as a matter of instinct, and can
train it through reflection. It can even have the strength to prevail
over religious immorality.
A story from Samoa (*) says that the religious practice of human
sacrifice was ended when the son of the Malietoa (one of the top-level
chiefs) saw that the woman he loved was going to be sacrificed, and
presented himself for the sacrifice in place of her. The Malietoa,
rather than sacrifice his son, banned human sacrifice thenceforth.
Whether or not this really occurred, the repetition of the story shows
that a human society can value conscience over religion, and it makes
an instructive contrast with the story of Abraham and Isaac.
* From Les religions du Pacifique et d'Australie by Nevermann, Worms,
and Petri.