The world is a dance in which good, descending from God, is disturbed by evil arising from the creatures, and the resulting conflict is resolved by God’s own assumption of the suffering nature which evil produces. C.S. Lewis, The Problem of Pain, 72
Evil as the absence of Good is a suitable description, but better to say, with Lewis, that Evil is a disturbance of Good. The surgeon’s scalpel used to murder, etc. Evil as the fault of humans is a suitable analysis, but better to use the more general term ‘creation’, so as to include non-human agency as well. Christian faith (like Lewis – i.e. Screwtape Letters) avoids both extremes of either disbelief in evil spirits or obsession with them.
But for theodicy, the salient point is that God is not the author of evil. God, however, as both Creator and Redeemer, is ‘responsible’ for both a) the creation of the world, which was always going to spoil itself, and b) the redemption of the world, which was always going to require the unspoiled Creator to unite to (and thus ‘drag up’ with him) the spoiled creation.