The kabbalistic thinkers were concerned with the nature and origin of evil. Many solutions were set forth. Influenced by the neoplatonists, the kabbalistic writers of Ma’arekhet ha-Elohut asserted that evil has no objective reality and is merely relative. Others suggested there must be a positive root of evil and death, which was balanced within the unity of the Godhead by the root of goodness and life.

The Zohar posits that the doctrine of evil emerged as a by-product of pure judgment. According to the Gerona kabbalists and the Zohar, pure judgment produced within itself the sitra achra ("the other side”), just as a vessel that is filled to overflowing spills its excess liquid on the ground. This "other side," separated from the World of Holiness and the Sefirot, became the residence of dark emanations and demonic powers. The distinction is critical. Though evil emerged from one of the attributes of God, it could not be considered an essential part of Him.

The Zohar avers that the sitra achra ("the other side") has ten Sefirot ("Crowns") of its own. Isaac ha-Kohen taught that the first worlds that were destroyed were three dark emanations, which perished on account of the strict judgment they contained. He maintained that the force of evil in this world comes from the emanation of Binah ("intelligence"). This emanation struggles with the emanation of Gevurah ("strength") from the beginning of creation.

The Zohar also asserts that the evil in the universe emerged from the "leftovers" of worlds that were destroyed. The power of evil is compared to the bark of the tree of emanation (a symbol that originated with Azriel of Gerona and has been employed in kabbalistic renderings of the Sefirot since then.) The emanation of the left is called the "outer tree." Another association interprets the "mystery of the Tree of Knowledge." The Tree of Life (representing loving-kindness) and the Tree of Knowledge (representing judgment) were bound together in perfect harmony until Adam came and separated them by eating of the fruit, thereby giving substance to evil, which had up to that time been confined. This materialization is known as the evil instinct (yetzer ha-ra). Adam’s act is called "the cutting of the shoots," and serves as the archetype of all the great sins mentioned in the Bible. The severing of the two trees symbolized the division of the Unity of God. Adam thus introduced separation "above and below" into what should have been a united world of creation. The power of judgment was summarily severed from the power of loving-kindness.

Many other perspectives were offered by the kabbalists to explain the existence of evil in the world. In some descriptions of the Zohar, evil is described as a natural waste product of an organic process and is compared to bad blood, a bitter branch on the tree of emanation, or the dregs from good wine. However, the identification of evil with physical matter never became a fully accepted doctrine in kabbalistic literature.

According to the Zohar there is a spark of holiness even in the domain of the sitra achra (the "other side”). The realms of good and evil are commingled, and the human mission is to separate them. This led to the theory of Nathan of Gaza who posited two lights in the Ein-Sof ("Infinite"): "the light which contained thought" and "the light which did not contain thought." These two lights are viewed antithetically: the first light was entirely active, the second light, passive; the first light contained the thought of creation, the second light had no share in creation. The second light, because of its passive resistance, became the ultimate source of evil in the act of creation. Thus, evil is the outcome of a dialectic between two aspects of the light of Ein-Sof itself. Its activity arises from the second light’s opposition to change.

It is an important conclusion. The concept that good and evil exist together in the supreme mind of God involved a rational approach to ascertaining the source of evil in a monotheistic system. How would evil survive in the World to Come? The commentator Nachmanides spoke of "the return of all things to their true essence." He meant by this the reascent of every created being to its source in emanation. This would negate the continued existence of evil or the evil inclination in man. Others held that the evil domain would survive as the place of eternal punishment for the wicked. The 13th century kabbalist Joseph Gikatilla stated that in the time to come "God will take the power of evil to a place where it will not be able to be malignant."

Those who upheld the doctrine that evil would once more become good claimed that the figure of Samael (the kabbalistic equivalent of Satan) would repent and be transformed into an angel of holiness, an act that would cause the disappearance of the "other side." Even Samael’s name would be changed. The letter mem (signifying death) would be dropped to leave Sa’el, one of the seventy-two Holy Names of God.