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.
