Well, there are two aspects to this answer.
The first is that evil was introduced by man, not by God. Evil exists then as a possibility, and indeed a potentiality, but is not 'present' as a reality until that fateful act. In the act the possibility of evil is realised, made real and actual, by the act itself.
The possibility, the potential, for evil, did not exist before man, because only a rational being can comprehend evil. Man created evil, not God – even though God knew of it, allowed for it, in giving man free will. The only 'real' act of evil is man's willing something other than what God wills for man. This was the freedom he sought, to taste freedom absolutely, to be not dependent on, and thus not beholden to, the will of God.
(One might argue this was simply the 'will to live' making a terrible mistake.)
But to exercise his own will, of his own volition, for good, assumes man knows the ultimate end, the ultimate good, of all things, which we do not – only God knows that. So at best an act of will on man's own behalf can only ever be 'a stab in the dark' – in a sense if he does the right thing it's more by luck than judgement, because he cannot judge accurately if he is not omniscient.
St Thomas Aquinas stated that no man wills evil for its own sake, rather he wills a 'lesser good' for his own sake, to suit his own end, which he can only guess at, and does not know.
Only by conforming his own will to the will of His creator can he 'know' certitude. Only the pure of heart can see God (as states the beatitudes). A pure heart wants for nothing other than what God wants of it.
(Christ asked St Thomas Aquinas, "what is it you want of me?" The Angelic Doctor replied, "You, Lord, only You.")
Once evil was introduced into the world man suffered its dreadful gravity, that continually drags him down from his birthright. In this sense evil is very real, it is a crippling wound, a corruption of his primordial nature.
Now, if man vanished overnight, if God struck every trace of man off the record, as it were, 'the evil that men do' would vanish with him. Evil is not what God does, and nor does nature – and in that sense it has no fundamental reality, because it is not willed by God as a reality.
(Our understanding of God would be very different if it were. At best we could only regard Him as capricious, as dangerous and untrustworthy – because we could not rely on any constancy on His part. He certainly would not be 'good' as we currently understand it, but spiteful, mean and cruel.)
So without man evil has no grasp of the world, no place in the world, no actuality in the created order. Man depends on God for his actuality. Evil depends on man – the serpent depends on man's act for its fulfillment.
The second aspect, which is a whole other subject, is that of fallen angels. This idea supposes a source of evil other than man, but that would require a discussion on the roots an development of Hebraic angelology, and perhaps creation itself.
Thomas