Baha'is do NOT hate Muslims, or members of any other faith. We are enjoined by our Founder, Baha'u'llah, to "Consort with the followers of every faith in fellowship and concord." We revere the Prophet Muhammad (peace be upon Him) as the Apostle of God, and the Holy Qu'ran as the Word of God.
The question might better be asked as, why do Muslims hate Baha'is? In every Islamic country, Baha'is are persecuted to one degree or another, from official slander to denial of civil and religious rights to imprisonment and execution. The usual reason is that we believe in a Messenger from God after Muhammad, and according to traditional interpretations, He was the Seal of the Prophets, and no Prophet will come from God until the "time of the end." The Qu'ran states that Muhammad is the last of the "nabi" or prophet, but He Himself was also a "rasul" or Messenger and Law-giver. Baha'u'llah is a "rasul" not "nabi" - He uses the term (in English) of "Manifestation of God" to describe the great Messengers Who began the world's religious traditions: Abraham, Moses, Jesus, etc. (peace be upon Them all).
there is also a common misconception that the bahai faith is a sect or branch of islam, which is also untrue.
As Jesus was born a Jew, Baha'u'llah was born into Islam and as Christianity is no longer part of the Jewish Faith the Baha'i faith is not part of Islam.