So, let me offer a different definition. I think we can both agree to this definition and it'll resolve this problem nicely.
I think we can safely decide that objective morality derives from God, thus, two otherwise similar actions can have different moral outcomes. Taking my sword and striking someone down because I want their wife is bad. Taking my sword and striking someone down because the Lord commands me isn't bad.
So from various Biblical examples we can otherwise assume that objective morality is going about one's life in a way that would please God, and being immoral would be going about one's life in a way that displeases God.
So, if we take it to be the case that morality derives from staying within God's limits or going outside of God's limits (morally speaking) we can establish that evil has existed forever and does not derive from human actions, it derives from God's expectations.
In other words, as soon as God makes a decision about how He wants humans to behave, He has created the conditions of possibility necessary for both Good and Evil. Eating the fruit from the tree was the first evil act by humans. Lucifer's rebellion was the first evil act by something not a human, but neither invented evil.
In this definition, the possibility for evil is created the moment God creates the possibility for good, since one is just the opposite of the other. This both allows us to make evil immanent to God in a way that doesn't make God evil, justifies the existence of evil in the world, and doesn't hurt any definitions of free will.
I hope I explained this well.