שִׁמְעוּ, כִּי-נְגִידִים אֲדַבֵּר; וּמִפְתַּח שְׂפָתַי, מֵישָׁרִים. (משלי ח:ו)
Listen, for I will speak noble things, and the opening of my lips shall be right things. (Mishlei 8:6)
Concerning the ba'al lashon hara, the slanderer, David HaMelekh writes in Tehillim 12:
ד יַכְרֵת יְהוָה, כָּל-שִׂפְתֵי חֲלָקוֹת-- לָשׁוֹן, מְדַבֶּרֶת גְּדֹלוֹת.
ה אֲשֶׁר אָמְרוּ, לִלְשֹׁנֵנוּ נַגְבִּיר--שְׂפָתֵינוּ אִתָּנוּ: מִי אָדוֹן לָנוּ.
4. May God cut off all smooth lips, the tongue that speaks great things.
The ba'al lashon hara is dangerous because he thinks that he speaks of 'great things'. He seeks to gain power and prominence by denigrating his fellow. He raises himself up by bringing others down. If he recognized the true majesty and dominion of God he could not be so glib and unconstrained. One who truly recognizes God's sovereignty would surely think it delusional to seek power. This is why the Rabbis say that the one who speaks lashon hara it is, "כאילו כפר בעיקר", as if he denies the most fundamental principle of the Torah: God's existence.
Lashon hara, slanderous speech, can be used as a lens with which we can focus on the mitzva of Sippur Y'tziat Mitzrayim. First of all, both are described as a kind of sippur. Sippur - normally translated: to tell a story, or to discuss - can not be so easily translated in these two cases. It does involve telling a story but in the sense that it takes in phrases like: "Now that's a story worth telling!" Both the m'sapeir (=one who tells a sippur) lashon hara, and the m'sapeir b'y'tziat Mitzrayim is motivated by greatness. While the m'sapeir lashon hara is motivated by the pursuit of his own pathetic greatness the m'sapeir b'y'tziat Mitzrayim is inspired by the greatness of God. The purpose of sippur y'tziat Mitzrayim is knowledge of God (Sh'mot 10):
א וַיֹּאמֶר יְהוָה אֶל-מֹשֶׁה, בֹּא אֶל-פַּרְעֹה: כִּי-אֲנִי הִכְבַּדְתִּי אֶת-לִבּוֹ, וְאֶת-לֵב עֲבָדָיו, לְמַעַן שִׁתִי אֹתֹתַי אֵלֶּה, בְּקִרְבּוֹ. ב וּלְמַעַן תְּסַפֵּר בְּאָזְנֵי בִנְךָ וּבֶן-בִּנְךָ, אֵת אֲשֶׁר הִתְעַלַּלְתִּי בְּמִצְרַיִם, וְאֶת-אֹתֹתַי, אֲשֶׁר-שַׂמְתִּי בָם; וִידַעְתֶּם, כִּי-אֲנִי יְהוָה.
1 God said to Moses, 'Come to Pharaoh, for I have made heavy his heart and the heart of his servants, in order that I would be able to place these, My signs in his midst. 2 And in order that you tell it in the ears of your children and grandchildren how I made a mockery out of Egypt, and My signs that I placed on them. And you will know that I am God.'
The m'sapeir lashon hara also speaks of what he thinks are wondrous and great things as Rambam writes at the end of Hilkhot Tumat Tzara'at in which he discusses the root cause of tzara'at: lashon hara:
קל וחומר לבני אדם הרשעים הטיפשים, שמרבים לדבר גדולות ונפלאות; לפיכך ראוי למי שרצה לכוון אורחיו, להתרחק מישיבתן ומלדבר עימהן, כדי שלא ייתפס אדם, ברשת רשעים וסכלותם.
[Miriam was afflicted with tzaraat for saying a very limited form of lashon hara against her brother Moshe...] All the more so, evil, foolish people who say big and wondrous things at length; therefore it is proper for one who wants to properly align his path to distance himself from their dwelling places and from speaking with them so that he will not get caught up with them, in the web of the evil and their foolishness.
The m'sapeir b'y'tziat Mitzrayim speaks at length of the truly wondrous and truly great deeds of the Almighty (Hilkhot Chametz uMatza chapter 7):
מצות עשה של תורה לספר בניסים ונפלאות שנעשו לאבותינו במצריים
It is a positive commandment from the Torah to tell of the miracles and wonders that were done for our forefathers in Egypt.
וכל המאריך בדברים שאירעו ושהיו, הרי זה משובח
Anyone who expands upon the matters that happened and that were - this is praiseworthy.
There is another halakha concerning how one must tell the story of y'tziat Mitzrayim (Hilkhot Chametz uMatza, chapter 7):
וצריך להתחיל בגנות, ולסיים בשבח.
It is necessary to begin with denigration and end with praise.
The halakha demands that we do this in two ways:
כיצד: מתחיל ומספר שבתחילה היו אבותינו בימי תרח ומלפניו, כופרים וטועין אחרי ההבל ורודפין עבודה זרה; ומסיים בדת האמת, שקירבנו הקדוש ברוך הוא לה, והבדילנו מן התועים, וקירבנו לייחודו. וכן מתחיל ומודיע שעבדים היינו לפרעה במצריים, וכל הרעה שגמלונו; ומסיים בניסים ונפלאות שנעשו לנו, ובחירותנו.
How [does one begin with denigration and end with praise]? Begin and tell that originally our forefathers in the days of Terach and before him were heretics and were swayed after vanity and chased after idolatry; and conclude with the true religion - that the Holy One Blessed is He brought us close to Him and separated us from the wayward and brought us close to His Unity.
And also, begin and inform that we were slaves to Pharaoh in Egypt and all the evil he caused us; and conclude with the miracles and wonders that were done for us and with our freedom.
What greater denigration can there be than being slaves and idolaters! The slanderer also denigrates, however, that is where it ends (Hilkhot Deot, chapter 7):
...הוא המספר בגנות חברו
...he tells of the scorn of his fellow
In contrast, speaking of the B'nei Yisrael's degradation is only a means to recognizing and praising God (Tehillim 113):
ז מְקִימִי מֵעָפָר דָּל; מֵאַשְׁפֹּת, יָרִים אֶבְיוֹן.
ח לְהוֹשִׁיבִי עִם-נְדִיבִים; עִם, נְדִיבֵי עַמּוֹ.
ט מוֹשִׁיבִי, עֲקֶרֶת הַבַּיִת-- אֵם-הַבָּנִים שְׂמֵחָה:
7. He lifts the pauper up from the dust, from the dungheap He raises up the needy,