The Ari [Rav Isaac Luria Ashkenazi] says that Emanation is pure Godliness, Briah (Creation) is mostly good, Yezira (Formation) half and half, and the Physical universe is mostly evil.

This explains a lot to me. We find people that start out sincere seekers of God and his will but fall. The reason I think is this. That every area of value has an opposite area of value. But the opposite can be no so much damaging as areas of value with more content. What I mean is that Logic is one area of value that is pure form and no content. The sentences: "A implies B. B implies C. If A is true, then C is true" are an example of the form is true but the content of each A B and C can be anything. zero content. Math has more content and is not just formal as Kurt Godel proved. Then up the scale you get to art and music with more content and less form. And the progression continues until you get to God--all content and no form. לא ראיתם כל תמונה ביום עמדכם בהר סיני "You did not see any form on the day you stood at Mount Sinai."

See this diagram which shows this [By Kelley Ross based on Leonard Nelson]

But every area of value has an opposite area. The Sitra Achra of that area. And since this world is mostly evil is easy to fall from holiness into the opposite area of value.