Despite the stupid assumption that G-d would not allow evil to exist, the fact is quite the opposite: the existence of G-d is absolutely essential to have evil.
Only G-d can define objective good and objective evil. Atheists find themselves in the predicament of defining for themselves "objective" evil in a world they hold is meaningless, when in a meaningless world objective evil could not exist.
Many people might cringe at the idea of G-d being responsible for evil, but G-d is responsible for everything. He did not have to create the universe but did so. By bringing into existence something other than himself G-d made evil possible. According to the midrash, already before Adam had even been created the earth rebelled against G-d when he commanded it to bring forth `eitz-peri `oseh peri ("trees of fruit bearing fruit") and it instead brought forth `eitz `oseh peri ("trees bearing fruit"). For this disobedience the ground was cursed at the expulsion of Adam. The same ground later tried to aid Cain by swallowing the blood of his slain brother Abel. There is also a midrash about the creation of the sun and moon in which the moon protested that "it is impossible for two monarchs to wear the same crown," for which it was reduced.
What all this means is that, since G-d cannot reproduce Himself, His creating something other than Himself is ultimately responsible for the existence of evil. Many chr*stians try to deal with this by positing that G-d created a perfect, sinless universe into which sin was introduced by Satan. This makes Satan a co-creator of the world, G-d forbid. But (contrary to chr*stianity), Satan is not a fallen angel but is merely doing his various jobs assigned to him by HaShem (as is illustrated in the Book of Job). Furthermore we know that G-d Himself created both the good and evil inclinations and placed them within Adam (this is the meaning of the two yods in Genesis 2:7 (Vayiytzer HaShem 'Eloqim 'et-Ha'Adam). Each of the two Yods is a reference to each of man's two yeitzers (inclinations)--good and evil. Add on top of this that the first human sins were committed by the perfect, unfallen Adam and Eve and it becomes obvious that G-d created the possibility of sin. Then on top of all that is the fact that everything that has ever happened or ever will happen is governed by G-d's Providence.
This we see that G-d is the sole creator of the world, not the "co-creator" with Satan, G-d forbid.
Atheists are stupid when they look at mass murder and other atrocities and then, to answer some perverted and self-defined sense of "justice," remove HaShem from the world. They are left with all the mass murders but they can no longer be said to be objectively evil.
Idiots.
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Christians don't believe that Satan is co-creator either.