If we describe something or someone as “pure evil” we are misunderstanding the nature of evil.
When confronted with particularly horrifying manifestations of the wickedness of the human race — things like the Holocaust, or child sex traffickers, we use phrases like “pure evil” to demonise the perpetrators and so break the connection between such acts and ourselves. We want to maintain that there is a difference not simply in degree but in kind between the worst people and ourselves. The Bible agrees that there are degrees of wickeness, but allows no distinction between “bad” people (them) and the rest (us):
There is no difference, for all have sinned and fall short of the glory of God [Rom 3:23]
Thinklings Weblog has a useful post on the nature of evil:
There is no such thing as pure evil. Evil is not an equal independent force, opposite of good. God existed before evil did, and he will exist after it ceases. Goodness is rooted in God. Evil, however, is simply a corruption of good. Biblical images of evil, point us back to good. Evil cannot exist independently of good. It only exists, by limiting or corrupting goodness.
The corruption of evil is so pervasive in our nature — what theologians call “total depravity” — that we are helpless to overcome it, and hopelessly estranged from God.
The good news is that through his death on the Cross, Christ Jesus has broken the power of evil over those who trust in him as their Lord and saviour:
if anyone is in Christ, he is a new creation; the old has gone, the new has come! [2 Cor 5:17]