It is because normal people cannot contemplate the world of the narcissist / psychopath /sociopath.

You have empathy, i have empathy.
We would never except a view from someone who said, we will kill any in our way, we will move forward with destructive measures even if it costs us our lives. "Our aim is to destroy the world, to bring about hell on earth" is what they say and do. But can you accept that?

On many other planets narcissists aren't allowed to exist.
They are terminated or they have their mind/personality wiped.
A good, functioning planet cannot allow for the existence of people that want to destroy the planet.

So, when someone says, we are going to destroy this planet, do you believe them, or do you think they are just kidding?

Also, you do no know what destroy this planet means.
You can't just blow everything up with nukes... doesn't work
You first have to make everyone believe that nukes exist, and then believe that they would be used, and that a country would want to destroy itself to destroy the world.
Basically, you have to get all the people to psychologically commit suicide

(like, have you looked into how many people weren't at the twin towers that day? Like, record low visitors)

So, we have to accept that there is evil.
And that these psychopaths and sociopaths intend to do evil.
Then we have to learn what evil looks like, and put a stop to it.

An example "No Child Left Behind"
Sounds good, but it was a most evil govern-cement school policy.