One difference between science and religion (of which there are many) is that some people can kill with the former as readily as others will kill for the latter. One similarity, on the other hand, is that in both, murder is committed far more for their fictions than for their facts. In thought, however, religion wars with human nature to establish what it defines as "moral" conformity, while science seeks to understand human difference, and as one destroys and punishes heretics (indeed, even Christ was a "heretic," after all), the other depends and promotes them.
In general, science and religion embody differing forms of belief and doubt that have battled in our minds ever since human beings evolved enough to pluck an apple from a tree and eat it. In general, the beliefs of both are that we are one and united, while the doubts of each are that we are separate and divided. The former perspective has historically been called “enlightenment,” from Buddha to Christ, and the light of the world, while the latter is often described as the shadow or the army of darkness.
Regardless of whether such darkness is described as “the devil and his minions” or simply a delusion of a deranged mind, however – regardless that is, of what vernacular we use to describe "extremism" as “evil” – those who commit such acts do so for a fictional belief that they serve some great power, or some great idea, that stands above humanity. As John Quincy Adams observed, "Power always believes... sincerely and conscientiously .. that it has a great soul and vast view beyond the comprehension of the weak, and that it is doing God's work, while it is breaking all of his laws."
Yet such a "belief" is a lie, for it sees humanity not as united, but as divided. It sees humanity, in other words, as standing for God or against God.
That “power” – whether it’s just an idea, or it’s their belief in their version of “God,” or a government, or a version of capitalism or democracy, or the like – can ultimately become their “God.” That belief/delusion then “possesses” them thereafter, like the fictional character Regan in the movie The Exorcist. And also like the movie, that idea can torture them into believing they must work to shape the world into something that conforms to their belief/delusion, in order to fulfill God's plan - no matter how many must be sacrificed to achieve it.
The problem with hating them for it, as natural as that may feel given the emotional devastation that such crimes inflict, is that it opens a trap door in our mind to allow the very same kind of evil/delusion that has poisoned theirs. The hardest thing to do, therefore, is to look down from such a cross and try to forgive them “for they know not what they do.” For if we fail to do this, then we may eventually become the ones who need the forgiving, and for the very same reason. Indeed, in all the destruction we might cause in response to such crimes, we will feel just as “righteous" in our defending ourselves as they did in defending their delusion. And this is how an eye for an eye, as Gandhi pointed out, makes the whole world go blind.