“The destructive effects of hatred are very visible, very obvious and immediate.  For example, when a very strong or forceful thought of hatred arises within you, at that very instant, it totally overwhelms you and destroys your peace of mind… it obliterates the best part of your brain, which is the ability to judge between right and wrong and the long-term and short-term consequences of your actions.  Your power of judgment becomes totally inoperable.  It is almost like you have become insane.  So this anger and hatred tends to throw you into a state of confusion which just serves to make your problems and difficulties so much worse.  Even at the physical level, hatred brings about a very ugly unpleasant physical transformation.”

Investigators, such as Dr. Redford Williams at Duke and Dr. Robert Sapolsky at Stanford University have conducted studies that demonstrate that anger, rage and hostility are particularly devastating to the cardiovascular system.  So much evidence has mounted about the harmful effects of hostility, in fact, that it is now considered a major risk factor in heart disease, at least equal to, or perhaps greater than the traditionally recognized risk factors such as high cholesterol or high blood pressure.”  

“For reasons such as these, hatred is compared to an enemy.  This internal enemy, this inner enemy has no other function than causing us harm.  It is our true enemy, our ultimate enemy.  It has no other function than destroying us, both in the immediate and long term.  This is very different from an ordinary enemy…” (who must eat and sleep, etc. and may one day be your friend.)

H.H. Dalai Lama, The Art of Happiness

For coping with anger or hatred, see  Buddhist Techniques for Overcoming Anger or Hatred.