Hatred and anger are the greatest poison to the happiness of a good mind.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“It would be a strange entertainment which consisted altogether of the imitations of hatred and resentment. If those passions are disagreeable to the spectator, they are not less so to the person who feels them. Hatred and anger are the greatest poison to the happiness of a good mind. There is, in the very feeling of those passions, something harsh, jarring, and convulsive, something that tears and distracts the breast, and is altogether destructive of that composure and tranquillity of mind which is so necessary to happiness, and which is best promoted by the contrary passions of gratitude and love.” source