Does the belief in evil actually make humans more likely to judge and react to others more harshly?

No.

People, in general, are pretty judgemental. I don't think it had anything to do with the belief in evil.

You can disbelieve gravity. Doesn't change the out come of falling down.

There is evil in the world and it comes in all colors sizes and gender.

Just to mention some famous ones, Hitler, PolPot, Stalin, Duvalier, etc. Lets try the US, Ted Bundy, Columbine, Va Tech, Boston marathon, OK City, 9/11.

It isn't a question of belief.

I disagree with both of the above posts. So called "evil" is actually a lazy form of judging what is misunderstood and hurtful.

If you break down each and every "evil" act to it's core, you would find mental illness, biological malfunctioning, environmental influences, etc... and would not ever find this "evil" state or object.

Because most (all) humans do not have the capacity to enter another's birth to end of life condition (body health and mental health) and experience they cannot truly judge accurately.

It gives humans an avenue to dismiss and dehumanize other humans without even having to consider the human condition that lead to the misunderstood and hurtful acts.