The origins of evil started with Lucifer. Problem solved
Isaiah 45:7 implies that God created evil.
The King James translation has been widely disputed and as I have already mentioned, the modern translations using "Disaster" and "Calamity" do not seem to be much different.
Remember also that All sin is evil but all evil is not necessarily sin.
We know that Lucifer was seduced by evil so what was this evil and where did it come from?
Looking strictly from scripture, this is pretty much all we have.
Simple logic tells us we understand light when we compare it with darkness.
Same goes with Good. We understand that Evil is the opposite of Good.
Prior to God declaring "Let there be light," was there light? Genesis tells that darkness existed prior to the light and of course we understand that darkness is the absence of light. The scriptures also say that "God is Light and in Him is no darkness at all" (Jn 1:5) so did darkness exist prior to God's work of creation? If darkness did not exist, was it yet theoretical?
Same goes with the Goodness of God. We understand that God is Good and everything that comes from God is good. Since everything comes from God, everything is good. Prior to creation, only God existed and since God is only good, evil did not exist. It was not possible for evil to exist but was it still "theoretical" as was darkness? Does not the goodness of God demand there be the possibility for evil by which God's goodness is compared?
You mentioned the "Free Will" of all God's creatures and prior to the fall, mankind had a PERFECT FREE WILL but how could he be tempted to do evil if evil did not exist? You may say Satan and this is a fair answer but you are just pushing the matter uphill right? How could Satan be tempted to do evil if evil was not already present?
It may be just an exercise in logic and critical thinking but our understanding of this affects our understanding of Hamartiology (The study of sin) as well as out understanding of human nature after the fall.
Evil did not originate with Lucifer but it did change from being a theoretical concept to a real, legitimate thing with Lucifer's rebellion.
If "Evil" existed and originated with Lucifer, would this not imply that this creature was somehow "Less than Good?" If God could create anything "Less than Good" would this not imply that God was somehow "Less than Good" in his character and attributes?