Rashi: You are a man important like myself.

Radak: I used to consider you (Achitofel) like myself regarding grandeur - so I honored you.

Malbim #1: Achitofel was a confidant of David. Achitofel spoke with him about a rebellion that can occur; David said, he need not fear this; if it happens, "we will go to Beis Elokim to pray" (verse 15), and quash the rebellion.

Malbim #2: This refers to verse 16 - "Yasi Maves Aleimo." Achitofel commanded his house [about after his death] and strangled himself. When death came to collect its debt from him (his Neshamah), and found that he had killed himself, it said, you are the angel of death like myself. You handed many over to death. You conspired with me to go to Beis Elokim at the time of rebellion. I.e. he counseled that also if David or his men will flee to Beis Elokim, where they did not kill anyone ("va'Ychazek b'Karnos ha'Mizbe'ach" - Melachim I, 1:50), they should kill them.

Radak #1: Someone greater than myself, and an officer over me.

Radak #2: One of my officers. He was among the king's officers and the head of his advisors - "va'Achitofel Yo'etz la'Melech" (Divrei ha'Yamim I, 27:33).

Radak citing Avos 6:3: My Rebbi and teacher.

Rashi: This is like Alufi (my teacher), an expression of "va'Eda'acha b'Shem" (Shemos 33:17); the Targum is v'Rabisach.

Radak: My friend. This is like "u'Meyuda'av" (Melachim II, 10:11). I used to inform you my opinion and secrets, like one does to his friend.