Evil is a slippery term. In it's conventional use, it draws on a vague psycological archetype of some 'thing' wholely negative, malicious, destructive. Religious justification is often sought for such a simplistic view of evil, because on even very shallow analysis it falls apart.
Dictators invariably believed at least initially they were doing good, or at least neccessary, things, and could be talked out of bad methods in principle. Psycopaths are ill, and can be healed.
Temptation and the fall, went against god's directions, but the fruit of the tree of knowledge can be seen as obviously neccessary, like recieving fire from Prometheus.
When we examine notions and examples of evil, and how and why it exists, we have to see it as behaviour that is pathological and needs healing for the benefit of the acted upon and the evil actor themselves.
A psycotherapist looks at impulses, that generate motivations. These are sources of vitality, when integrated and properly harnessed. Can impulses at core be evil? Possibly, in a sick person. But they arise from our biological needs, our desire for health and wholeness and wellbeing. The therapeutic perspective is that people can be healed, meaning wholesome impulses generated. Our fundamental nature cannot be aimed against itself, that can only have been aquired.
Actual evil actions are capable of being redeemed, like people are capable of being healed. Where does the energy in the motivation come from? In so far as it is evil, it is vitality aimed against itself. Vitality, rooted in our biological being, seeks to regenerate itself, to be reclaimed into positive cycles.