Re'ei - "See, I set before you this day a blessing and a curse........"

Evil alternatives exist to allow for free choice. If there was only good in this world - no chance for a person to behave in a questionable manner - he couldn't freely choose to do good; he would be forced to do good by default. And there would be no room for reward and punishment.

Wrong exists only to allow a person to choose right. Evil is nothing but a means of improving our Divine service, to push the person toward the correct path. Evil is not a curse, but a merit that enables us to succeed and prevail. Knowing this, gives us the ability and strength not to be intimidated or overwhelmed by the bad.