There are three main reasons for the attack on the American Embassy and they are found written in the Holy Qur'an.
1) Muhammand's message is universal: Surah 7:158
YUSUFALI: Say: "O men! I am sent unto you all, as the Messenger of Allah, to Whom belongeth the dominion of the heavens and the earth: there is no god but He: it is He That giveth both life and death. So believe in Allah and His Messenger, the Unlettered Prophet, who believeth in Allah and His words: follow him that (so) ye may be guided."
PICKTHAL: Say (O Muhammad): O mankind! Lo! I am the messenger of Allah to you all - (the messenger of) Him unto Whom belongeth the Sovereignty of the heavens and the earth. There is no Allah save Him. He quickeneth and He giveth death. So believe in Allah and His messenger, the Prophet who can neither read nor write, who believeth in Allah and in His Words, and follow him that haply ye may be led aright.
SHAKIR: Say: O people! surely I am the Messenger of Allah to you all, of Him Whose is the kingdom of the heavens and the earth there is no god but He; He brings to life and causes to die therefore believe in Allah and His messenger, the Ummi Prophet who believes in Allah and His words, and follow him so that you may walk in the right way.
2) No religion but Islam:Surah 9:33
YUSUFALI: It is He Who hath sent His Messenger with guidance and the Religion of Truth, to proclaim it over all religion, even though the Pagans may detest (it).
PICKTHAL: He it is Who hath sent His messenger with the guidance and the Religion of Truth, that He may cause it to prevail over all religion, however much the idolaters may be averse.
SHAKIR: He it is Who sent His Messenger with guidance and the religion of truth, that He might cause it to prevail over all religions, though the polytheists may be averse.
3) One must revert (convert) to Islam. There is no choice.
Muslims are to inflict terror until the non-believer(s) revert (convert) back to Islam: Surah 3:151
YUSUFALI: Soon shall We cast terror into the hearts of the Unbelievers, for that they joined companions with Allah, for which He had sent no authority: their abode will be the Fire: And evil is the home of the wrong-doers!
PICKTHAL: We shall cast terror into the hearts of those who disbelieve because they ascribe unto Allah partners, for which no warrant hath been revealed. Their habitation is the Fire, and hapless the abode of the wrong-doers.
SHAKIR: We will cast terror into the hearts of those who disbelieve, because they set up with Allah that for which He has sent down no authority, and their abode is the fire, and evil is the abode of the unjust.
The above passages were taken from the Holy Qur'an and were carried out by Muslim Terrorists when they attecked the American Embassy in Libya. This is the only reason the American Embassy was attacked and Americans were murdered.