Someone who speaks lashon hara against his fellow transgresses the positive mitzvah (Devarim 24:9), "Remember what Hashem your G-d did to Miriam in the way, etc," which warns us what happened to Miriam, even though, when all is said and done, she only spoke against her brother whom she loved like herself and had endangered herself to save him from the water, and when all is said and done only compared him to other prophets and did not actually speak bad of him. Also, she did not speak in public but only before Moshe Rabeinu. Nonetheless, she was punished with leprosy. How much more will those who speak evil of people be punished.
In addition to this, one transgresses the positive command (Vayikra 19:18), "Love your fellow like yourself," because someone who speaks or accepts lashon hara certainly does not love his fellow, since a person does not like to relate his own faults and his fellow certainly does not want people to speak of his shortcomings.
