There are a few reasons as to why derogatory speech is considered such a severe sin: A. It arouses accusation in heaven towards the Jewish people, and causes death and destruction in this world. B. A person who defiles his mouth with speaking the forbidden, all things sacred hence forth are prevented from ascending heavenward and there sanctity is thus diminished. C. A so called ‘light’ sin that is repeated many times becomes more severe, just like a thin string that is connected to many thin strings becomes a thick rope. How much more is this so regarding forbidden speech which is considered a most severe sin, the fact that it treated lightly causes the damage to be tenfold.
ג׳ בתשרי ה׳תשפ״ב
Every moment that prerson restrain him mouth he acquiting to the Hidden Light, that there is no Mal'ach and creation to be able Imagine.(Hagra letter) 1