An angry person will easily transgress the prohibition of “Do not hate your friend in your heart.” Instead of fulfilling the mandate of Chazal to resolve differences pleasantly, he’ll harbor hatred and resentment in his heart. (ibid., 1, 23) The sin of baseless hatred is different than other sins in that a person is constantly transgressing it while he harbors this hatred in his heart. Sometimes a person will feel this hatred for a month, a year or even longer, until it’s impossible to calculate how many sins a person has accumulated! (Ahavas Yisrael, chapter 2)
