First, both Muslims and Christians are human beings, imperfect and sinful human beings (both religions agree on that). Second, and just to make things worse, both Islam and Christianity have, at times, been official state religions, meaning that states acted in the name of their religion. As a result, there have been plenty of moments in history where Christians and Muslims fought each other. Yes, it is true that Muslims and Christians often did live in peace side by side, but unless one is a total bigot and ignoramus, it is simply impossible to ignore the fact that Christians and Muslims also waged war, persecuted and mistreated each other, sometimes viciously.