This question and others (such as “Why do we suffer?”) have plagued humankind since time immemorial.
There are Kabbalistic discussions about the existence of evil, but the most powerful and life-changing answer is that evil exists in order to offer contrast with good.
The existence of evil enables us to make a choice. This choice is actually necessary for our life mission.
With evil, free-will exists. Without it, we may as well be programmed robots.
Read the rest of this post at A Little Bit of Soul, my new blog for readers of all backgrounds, at PsychCentral.com. Please share.