The primary prohibition against speaking loshon hora is: Do not go as a gossip-monger among your people (Vayikra 19:16). However, the Chofetz Chaim demonstrates that this sin can involve the transgression of numerous positive and negative commandments. The baal loshon hora, habitual speaker of loshon hora, can easily accumulate a multitude of transgressions at a rate unparalleled by any other sin. That is why the Sages say that speaking loshon hora is worse than even the most severe sins.
Furthermore, it is virtually impossible for the habitual speaker to ask forgiveness of everyone who was affected by his sinful speech: thus, he will find it difficult if not impossible to achieve full repentance for his sins.
The Sages caution us to avoid associating with a baal loshon hora, and not to live in the vicinity of such people.
Sefer Chofetz Chaim, 8 Cheshvan, page 124