In almost the same way you know what your grandmother looks and sounds like, you know what Bruce Willis looks and sounds like.
It is true that some liberals and humanists, along with the laid-back Danes, deny the existence of evil. This is largely because they regard the word 'evil' as a device for demonising those who are really nothing more than socially unfortunate.
We will not learn how to live together in peace by killing each other's children.