When we stop to think about it, evil wouldn’t exist in the world if it wasn’t for us. All the forms of evil we know were created and fashioned by beings other than God. There is nothing of what God wills that is evil – including evil’s existence. If man (or angels) did not exist there would be no such thing as evil. But as long as beings have the power of choice (i.e. the power to love or hate) then evil is a necessity because for a choice there must be at least two options. Thus we see the necessity of both good and evil – the necessity of choice – because we cannot love without choice and loving is always choosing the good.
The glitch is that we so often have trouble recognizing the good as good and the bad as bad. In fact, through ignorance and/or rationalization we choose what is bad, wrong, or evil fully welcoming it as good.
Evil’s existence is denied by the existence of God, yet we attempt to give it existence despite God. Thus we deny the good when we affirm evil. Why are we permitted this power over the good? Did God not see that evil would come from creation? But, of course, this was not His choice even though He did foresee it.
The Kingdom of God is the kingdom of love and there can be no love without choice. The choice not to love is real and it breeds evil. But it is permitted – by God and by us – because without the two choices we cannot choose love. The only place love can exist without evil is in eternal union with God. If we reach that, we have no need of choice – we’ve already chosen!