It is true, no matter how tough of a pill it is to swallow, Islam is a descendant of Christianity.  The separation between the two religions fundamentally is differentiated by which recognizes as being the last messiah.   We Christians believe Jesus Christ was the one and only Messiah and Islam believes there were many, the last of which was Muhammed.

No one can deny that there was some crazy shit in the Old Testament, God instructing people to kill people, commit adultery, and even steal.  But that can’t be right because of the Ten Commandments, right? No, it is.  You see, the priests of this time could take liberties with the Bible and they would often change or make exceptions to certain rules when it became convenient for them. But then something super-human really did happen.  This guy, with a huge following of people documenting his life, came in, uncorrupted and gave the real teachings of God.  He taught us to love our neighbors, turn the other cheek, forgive, and use violence only as a last resort for survival.

In contrast, Muhammed taught entirely the opposite.  He preached that anyone who believed differently than his followers were to be executed.  He taught that you were to seek vengeance and retribution for transgressions and even taught his followers to preempt certain actions and to lie, cheat, and steal where necessary to promote Islam.  This is the crossroads at which two fundamentally similar religions became such fundamental enemies.

While Islam is inherently a vile system of beliefs; you must remember that it is still partially based on Christianity.