"Where does hatred come from?"

Dalai Lama : "That is a question which requires long hours of discussion. From the Buddhist viewpoint, the simple answer is that it is beginningless. As a further explanation, Buddhists believe that there are many different levels of consciousness. The most subtle consciousness is what we consider the basis of the previous life, this life, and future lives. This subtle consciousness is a transient phenomenon which comes about as a consequence of causes and conditions. Buddhists have concluded that consciousness itself cannot be produced by matter. Therefore, the only alternative is to accept the continuation of consciousness. So that is the basis of the theory of rebirth.
Where there is consciousness, ignorance and hatred also arise naturally. These negative emotions, as well as the positive emotions, occur right from beginningless time. All these are a part of our mind. However, these negative emotions actually are based on ignorance, which has no valid foundation. None of the negative emotions, no matter how powerful, have a solid foundation. On the other hand, the positive emotions, such as compassion or wisdom, have a solid basis: there is a kind of grounding and rootedness in reason and understanding, which is not the case with afflictive emotions like anger and hatred.
The basic nature of the subtle consciousness itself is something neutral. So it is possible to purify or eliminate all of these negative emotions. That basic nature we call Buddha-nature. Hatred and negative emotions are beginningless; they have no beginning, but there is an end. Consciousness itself has no beginning and no end; of this we are certain."