“Why does evil exist?” you ask?
Wellllllll…..
Evil Exists, because in order to preserve the existence of the universe, there needs to be balance. To life there is death, to joy there is grief, to success there is failure, and, alas; to good there is evil. None can exist without the other. If one did exist without the other, then it would lose its meaning. And ultimately, life does have meaning. How would anyone recognize joy if they never knew the opposite? How could they cherish their own state of happiness if they didn’t know there was an alternative? How could they treasure their own life if they never had to let it go? Without opposites, life is only half of itself – and that half becomes a place of numbed indifference. Therefore, in my opinion, evil exists because good does.
— Written by a completely non-subjective person.
theskywillfall 
Simply, clearly, and elegantly put-well done!
Cvet