Sometimes a person speaks Loshon Hora because the Yetzer Hora convinces him that it is allowed.
Sometimes, the Yetzer Hora actually convinces a person that it is a mitzvah to speak Loshon Hora. A mitzvah! Imagine!
So you see, we must be very careful to protect ourselves from the Yetzer Hora. The only way to do this is to learn the laws of Loshon Hora and to know them well. Only then will we know what is allowed and what is forbidden, what is a mitzvah and what is a sin!