Here is my theory:
Some religious people in this world, you may have noticed, are deeply possessed by anger
and a compulsion to commit violence
But this goes against one of the most universal and enduring religious sensibilities, that religion ought to be a force for peace
So why is this, you may ask?
What could so fill a person with rage and bile as to make them murderous, even as to total strangers, even as to children, and sometimes even as to some of those who express, in broad strokes, the same religious views as themselves? And more than murderous to some, what could desirous of inflicting terror
on all, or at least on massive numbers of mankind? There is nothing which is "true" which can inspire such desire.
And so the obvious answer is that deep down, deep in their hearts, perhaps deep, deep, deep in their subconscious minds, they know their specific religion
(at least, their understanding of it) to be false. Some part of them simply knows they've been brought up believing bullshit. They know in their unspoken gut they've been played for fools all their lives and even now are parading around like idiots for civilized people to laugh at.
In order to carry out such madness
on religious grounds, the believer must surely first be driven mad, mad by attachment to a maddeningly false faith, mad by the confession at some level of their being that they have been suckered into an embarrassingly, humiliatingly stupid religious framework.
And perhaps this deep and self-loathing internal knowledge arises from a recognition, that any religion which commands killing
others for differing beliefs, for absence of beliefs, for harmless theological differences of opinion, betrays so weak a confidence
in any objective rightness that it cannot be anything by embarrassingly stupid to have been raised in it.
It is, thusly, not the "believer" who grasps the knife, the gun, the bomb, to harm for faith; no, it is the unbelieving heart which so furiously disdains its holder's religion as to deny peace and love themselves.