"3 things a person is punished for in this world and has no world to come: idolatry, sexual immorality, and murder. And lashon hara corresponds to all of them."
Why is lashon hara (evil speech) compared to the three most severe sins? Humans are unique creatures. Our speech is an expression of our capacity for abstract thought, but it is also a tool for our instincts.*
There is an interesting psychological phenomenon called "sublimation." About.com says it is a defense mechanism where we channel unwanted feelings into more socially acceptable forms.
We have aggressive impulses and sexual impulses. We don't act them out overtly. But they are expressed in our speech. Much of our lashon hara is a way of enjoying sexual scandals and a way of hurting other people. Of course, we would not actually engage in these violations. We are not so base. However, this statement of Chazal is urging us to see that the underlying impulses that drive lashon hara are often sublimations of these aggressive and sexual urges.
Idolatry is a particularly interesting case. By observing when I speak lashon hara, I noticed something: I frequently have been insulted or hurt or put down in some way. I feel inferior from this interaction. So what do I do? I call up a friend and get her to agree that the person who made me feel inferior is wrong, and *I* am superior.
This is basically a framework that I invented in my own head. There are imaginary scales in my mind. If I feel bad, I have to balance the scales by getting a friend to agree with me that the person who made me feel inferior is actually inferior, and I am superior. In real life I have achieved nothing. I have not addressed the conflict with the person. I have not put things in perspective (maybe the other person was having a bad day; sometimes I behave like that, etc.). All I have done is spoken negatively about someone to someone else, they have agreed with me---and poof! I feel better. I have constructed an artificial reality inside my own mind about how people are evaluated. But it's not enough to leave it in my own mind; I must get social approval of this construct by getting a listener to agree with me.
This has an uncanny resemblance to Avoda Zara. A major aspect of idolatry is that it is a manmade construct. Judaism's goal is for humans to relate to the world as it actually IS, how Hashem designed it.** Idolatry, on the other hand, is a human invention that seeks to control destiny. If you don't want to get sick, or you want to make money, you have to sacrifice to a deity. But who invented the deity? Humans.
Lashon Hara does not accept the divine reality that a person is superior or inferior based on values and deeds. Lashon Hara is based on the human contrivance that someone who insults ME has appointed me as inferior and the only way I can fix that is if other human beings appoint ME as superior and HIM as inferior. Idolatry is an attempt to bend reality to your wish. So is Lashon Hara.
As a side note, Lashon Hara is a particularly insidious way of dealing with interpersonal conflict. Instead of dealing directly with the person you have conflict with, you feel bad about the interaction and then you call up a friend and complain about it. Lashon Hara prevents us from figuring out how to have healthier dynamics with people who elicit feelings of inferiority in us.
* Bereshis 2:7 says "He blew into his nose a living soul and Man became a live spirit (nefesh chaya)." Onkelos translates the nefesh chaya as "a speaking spirit," meaning that Man was given the ability to speak. Rashi comments that animal and Man are both called nefesh chaya, but Man additionally has knowledge and speech.
** "Tamim tehiye im Hashem Elokecha (Devarim 18:13), you should be complete with Hashem your God," i.e. we should be okay with how Hashem designed the world.