When you learn Biblical Hebrew, one of the most important skills that you can acquire    is the ability to understand the meaning of the nominal system. As a Biblical Hebrew teacher the first steps with the Hebrew are teaching simple words like father or peace.
 

Absalom, who was David's son, wasn't the father of peace as referred by his name. He rebelled against his father constantly after David didn't take care of his son Amnon, who raped and tortured Tamar.  Absalom didn't forget the rape of his sister, and he even called his girl by her name as we can read in 2 Samuel 14:25-27-

"וּכְאַבְשָׁלוֹם, לֹא-הָיָה אִישׁ-יָפֶה בְּכָל-יִשְׂרָאֵל--לְהַלֵּל מְאֹד:  מִכַּף רַגְלוֹ וְעַד קָדְקֳדוֹ, לֹא-הָיָה בוֹ מוּם.  וּבְגַלְּחוֹ, אֶת-רֹאשׁוֹ, וְהָיָה מִקֵּץ יָמִים לַיָּמִים אֲשֶׁר יְגַלֵּחַ, כִּי-כָבֵד עָלָיו וְגִלְּחוֹ; וְשָׁקַל אֶת-שְׂעַר רֹאשׁוֹ, מָאתַיִם שְׁקָלִים בְּאֶבֶן הַמֶּלֶךְ.  וַיִּוָּלְדוּ לְאַבְשָׁלוֹם שְׁלוֹשָׁה בָנִים, וּבַת אַחַת וּשְׁמָהּ תָּמָר; הִיא הָיְתָה, אִשָּׁה יְפַת מַרְאֶה"
 

"But in all Israel there was none to be so much praised as Absalom for his beauty: from the sole of his foot even to the crown of his head there was no blemish in him. And when he polled his head, (for it was at every year's end that he polled it: because the hair was heavy on him, therefore he polled it: he weighed the hair of his head at two hundred shekels after the king's weight. And unto Absalom there were born three sons, and one daughter, whose name was Tamar: she was a woman of a fair countenance."
 

After he didn't speak with his father for two years, the rebel seemed to come to an end, but Absalom decided that he and not David will be the judge of Israel as we can see in 2 Samuel 15:4-6-

"וַיֹּאמֶר, אַבְשָׁלוֹם, מִי-יְשִׂמֵנִי שֹׁפֵט, בָּאָרֶץ; וְעָלַי, יָבוֹא כָּל-אִישׁ אֲשֶׁר-יִהְיֶה-לּוֹ-רִיב וּמִשְׁפָּט--וְהִצְדַּקְתִּיו.  וְהָיָה, בִּקְרָב-אִישׁ, לְהִשְׁתַּחֲו‍ֹת, לוֹ; וְשָׁלַח אֶת-יָדוֹ וְהֶחֱזִיק לוֹ, וְנָשַׁק לוֹ.  וַיַּעַשׂ אַבְשָׁלוֹם כַּדָּבָר הַזֶּה, לְכָל-יִשְׂרָאֵל, אֲשֶׁר-יָבֹאוּ לַמִּשְׁפָּט, אֶל-הַמֶּלֶךְ; וַיְגַנֵּב, אַבְשָׁלוֹם, אֶת-לֵב, אַנְשֵׁי יִשְׂרָאֵל."

"Absalom said moreover, Oh that I was made judge in the land, that every man which hath any suit or cause might come unto me, and I would do him justice! And it was so, that when any man came nigh to him to do him obeisance, he put forth his hand, and took him, and kissed him. And on this manner did Absalom to all Israel that came to the king for judgment: so Absalom stole the hearts of the men of Israel."

David saw that he can't deal with the admiration that the people of Israel had to his son, He ran away, left some of the wives in Jerusalem. However, he sent a spy to Absalom's camp as we can read in 2 Samuel 15:37-

"וַיָּבֹא חוּשַׁי רֵעֶה דָוִד, הָעִיר; וְאַבְשָׁלוֹם, יָבוֹא יְרוּשָׁלִָם"
 

"So Hushai David's friend came into the city, and Absalom came into Jerusalem."
 

At this point, like every leader has, Absalom was sitting in Jerusalem, and he needed an advice what to do. Hushai and Ahithophel were there to help him with his decisions. Ahithophel gave Absalom two advices: the first is to sleep with his father's wives publicly as Nathan the prophet promised Davis after he caused the murder of Bath-Sheba's husband and slept with her.

 
The second advice is written in 2 Samuel 17:1-4-

" וַיֹּאמֶר אֲחִיתֹפֶל, אֶל-אַבְשָׁלוֹם:  אֶבְחֲרָה נָּא, שְׁנֵים-עָשָׂר אֶלֶף אִישׁ, וְאָקוּמָה וְאֶרְדְּפָה אַחֲרֵי-דָוִד, הַלָּיְלָה.  וְאָבוֹא עָלָיו, וְהוּא יָגֵעַ וּרְפֵה יָדַיִם, וְהַחֲרַדְתִּי אֹתוֹ, וְנָס כָּל-הָעָם אֲשֶׁר-אִתּוֹ; וְהִכֵּיתִי אֶת-הַמֶּלֶךְ, לְבַדּוֹ.  וְאָשִׁיבָה כָל-הָעָם, אֵלֶיךָ:  כְּשׁוּב הַכֹּל--הָאִישׁ אֲשֶׁר אַתָּה מְבַקֵּשׁ, כָּל-הָעָם יִהְיֶה שָׁלוֹם.  וַיִּישַׁר הַדָּבָר, בְּעֵינֵי אַבְשָׁלֹם, וּבְעֵינֵי, כָּל-זִקְנֵי יִשְׂרָאֵל."
 

"Moreover Ahithophel said unto Absalom, Let me now choose out twelve thousand men, and I will arise and pursue after David this night: And I will come upon him while he is weary and weak handed, and will make him afraid: and all the people that are with him shall flee; and I will smite the king only: And I will bring back all the people unto thee: the man whom thou seekest is as if all returned: so all the people shall be in peace. And the saying pleased Absalom well, and all the elders of Israel."

The advice seemed to be good as all the people of Israel were pleased including Absalom. However, if the advice was good, why is the saying " Ahithophel Advice'" considered today to be a bad one?
 
First of all, the advice wasn't accepted by Absalom because The Lord commanded to do it in order to keep David alive. Second, Absalom asked for another advice from Husai, who said that Ahitophel's advice wasn't good. Even if the advice of Ahitophel was good, when Absalom asked for another advice that means that he wasn't sure enough, or maybe he changed his mind, as we can read in 2 Samuel 17:14-

" וַיֹּאמֶר אַבְשָׁלוֹם, וְכָל-אִישׁ יִשְׂרָאֵל, טוֹבָה עֲצַת חוּשַׁי הָאַרְכִּי, מֵעֲצַת אֲחִיתֹפֶל;  וַיהוָה צִוָּה, לְהָפֵר אֶת-עֲצַת אֲחִיתֹפֶל הַטּוֹבָה, לְבַעֲבוּר הָבִיא יְהוָה אֶל-אַבְשָׁלוֹם, אֶת-הָרָעָה."
 

"And Absalom and all the men of Israel said, the counsel of Hushai the Archite is better than the counsel of Ahithophel. For the LORD had appointed to defeat the good counsel of Ahithophel, to the intent that the LORD might bring evil upon Absalom."

Today, as I mentioned before, the advice of Ahitophel is an expression of a bad advice unlike in the Hebrew Bible, as it was good according to the words of the author. The lesson from this story is clear: we need to speak and share our thoughts with our friends. Let us just pay attention to that our advices won't be bad as Ahitophel's!