@Narkissos
You are getting into serious heresy here...
What I mean is that, in my view, the nature of evil is one of the most essential theological issues that channel the rise of monotheism as we know it today.
During the first centuries A.D. the development of Christianity virtually defines itself in opposition to various dualist heresies (notably gnostics), and later orthodoxy tried to eradicate every single group that had these dualist tendencies (manichaeans, the gnostic groups, later cathars etc.). The sheer hatred directed against these groups is truly frightening.
Augustine defines evil as a privation of goodness - evil is defined away, so to speak. Aquinas comes up with 'functional evil': evil that, in God's grand scheme, ultimately will bring about good.
On a more global scale, the historical development of the theology of evil is also interesting:
Zoroastrianism - Cosmical Dualism (Ahuramazda is wholly good and Ahriman is wholly bad).
But in some branches, like Zurvanism, Ahuramazda and Ahriman are twins, both sons of Time, two sides of one coin. This tends to Divine Dualism (dualism inside the divinity)
Early Judaism - more like an ancient henotheistic faith, I would say. Yahweh as warrior stormrider God, historically taking the place of Ugaritic 'El. Neither 'El nor Yahweh are wholly good (as demonstrated in the Baal Cycle and many parts of the OT). But, like the Greek Gods, the question about moral goodness is not really important. Gods are just super-men in the sky. They behave like us, go to war, have sex (also see the Baal Cycle) etc.
Persian Judaism - Divine Dualism, showing influences of Zoroastrianism (for example: http://www.users.globalnet.co.uk/~slocks/buckner/evil.html)
Late Judaism and Christianity - God is wholly good. Man, as created in God's Image, is also good. Sinning is missing the mark, evil is a missing of goodness.
Islam - God is wholly good.
(And whenever they commit an indecency they say, 'We found our fathers practising it, and God has commanded us to do it.' Say: 'God does not command indecency; what, do you say concerning God such things as you know not?' Sura. 7:28)
Regards,
Deus Mauzzim
(of the Dualist Class)