Because you can't reason with someone who doesn't employ reason. That doesn't mean they won't kill you - it just shuts down one avenue by which you might talk them out of it.

But religious people can be reasonable and atheist can be unreasonable.

You can't reason about first principles. What first principles someone chooses as their "axioms" doesn't alter their ability to be reasonable. Materialism or empiricism are just as much a random choice for a base principle as deism or theism or many others.

A lot of the evils that the hardcore atheist crowd (Dawkins, Sam Harris, that crowd) ascribes to religion is oftentimes much better understood as imperialism and other purely political ambitions couched in what was the most common first principle of the time. The desire of European kings (including the Pope) to hold Jerusalem and later Constantinople were much better understood as a desire to control trade and expand their territory/influence rather than some deeply seated religious fervor, just for the example of the crusades. And for things like the inquisition, we can see today as well plenty of largely secular demonization and oppression of marginalized groups.