Proverbs 8:13 is one of the definition verses of the Bible, along with "sin is the transgression of the law" (I John 3:4, KJV) and "the love of God [is] that we keep His commandments. And His commandments are not burdensome" (I John 5:3), among others. Here, the fear of the Lord is defined as "hating evil."
In the Bible, "evil" is used in a wide variety of ways, but as we might expect, its basic meaning is simply "bad" or "negative." It appears in both the passive and active senses. When used passively, it describes distress, misery, misfortune, calamity, or repulsiveness. Proverbs 8:13, however, does not express the passive form of evil, but the active form, which is used in two ways in the Bible. The first can be defined as "what is wrong with regard to God's original and ongoing intent," while the second is narrower in scope: "what is detrimental in its effects on mankind."
People are most familiar with the second definition. When we think of evil, we typically imagine something that is purposefully injurious or intentionally unkind. It is not merely bad in the sense that a hurricane may be bad; it is more than merely unpleasant, but rather terrible by someone's design. In this definition of evil, there is intent to harm—or at the very least, ambivalence toward harm done to another. Evil does not care if harm is done.
In his book, People of the Lie—subtitled "The Hope for Healing Human Evil"—Dr. M. Scott Peck provides a simple yet profound definition of evil: "that which does harm to life or liveliness." The book is about "malignant narcissism": self-centeredness so extreme and pervasive that those possessing it continually injure others around them, not with physical wounds, but with subtle assaults on their emotional or spiritual well-being. This evil cannot be observed directly—the malignant narcissist is a master of deception—but can be seen only in its effects on others, in subtle violence perpetrated against the human spirit in others. Even as these people are doing harm to life and liveliness, they are putting on a pretense of righteousness and piety, terrified at the thought that others might see them as they truly are or that they might actually have to face themselves.
This second way that "evil" is used in the Bible—"what is detrimental in terms of its effects on mankind" or "that which does harm to life or liveliness"—can be quite subjective, thus the Bible also defines it as "what is wrong according to God's intent." A common description in the Old Testament is that a certain person or group "did evil in the sight of the LORD." This description is key because the people did not consider their deeds to be evil. In their view, they were harmless acts. Nobody was getting hurt, and nothing detrimental occurred (that they could see), so they did not consider their behavior to be evil. But what they did was evil—in God's sight.
Israel and Judah justified blatant idolatry and even child sacrifice by saying that they were not doing any harm, or that the harm it might do to the child was insignificant compared to the "greater good" that they believed would come from the sacrifice. The same justification is used for the practice of abortion today.
Israel did not consider temple prostitution to be harmful either, but in the eyes of God—the only eyes that see objectively—what they did was evil. It was evil not just in terms of going against God's intent; it went against God's intent because it was injurious to those involved in it, even though they could not see it. In their myopic pride, they were unable to see that what they were involved in would ultimately bear horrible fruit. So God had to define right and wrong, good and evil, because man is so shortsighted that he often cannot see what will cause harm to himself or to a neighbor.
Halloween is a good example of this, for it is nothing short of the glorification of evil. Its roots go back to the Celtic festival of Samhain, who was the "lord of the dead." It was a boiling mixture of drunkenness, revelry, licentiousness, vandalism, treachery, superstition, anarchy, and rank demonism. Today, this festival is dressed up in a creative costume and dubbed "fun for the kids," but its essence is the same. The world calls it "harmless fun," but it is obvious from Scripture that it is "evil in the sight of the LORD." The seed from which Halloween grew was paganism—really just a softer term for "demonism"—and if the seed is evil, the fruit will also be evil, even if presented in a "fun" way. Yet, many people enjoy this annual dose of witches, vampires, and werewolves. They have no problem indulging in the occult, if only in their imaginations.
However, Proverbs 8:13 says that those who fear God instinctively and earnestly loathe those things that do harm to life and liveliness, even if the harm is not immediately apparent. The elements of Halloween, no matter what guise they are in, are contrary to eternal life with God. If we fear God—if we respect Him and what He stands for—then we also oppose all that He is against, which certainly includes anything associated with "the evil one" or his subservient "evil spirits."