"There is no God but Allah, and Muhammed is His Prophet."

The problem is that is you don't believe it, the Koran, which Muslims believe to be the literal, perfect word of Allah and not subject to "interpretation" for more modern realities and acquired secular knowledge, instructs Muslims to either convert you or kill you. And if they die in the process they go straight to Paradise.
Sounds pretty bloodthirsty to me. When Mohammad had the unarmed, defenseless Jews of Medina exterminated, he was simply being true to his new faith. Likewise for Osama bin Laden on Sep. 11, 2001.

It's true that the violent actions and mindless intolerance of Muslim fundamentalists are no worse than those of, say, the Christian Inquisition. And millions today are still suffering from the inanities of Christian religious leaders, as overpopulated and AIDS infested peoples are denied proper sexual education and condoms, for example. As a belief system based upon faith not reason, all religion is bad, it's just that Islam, which is so maddenly impervious to the softening, moderating influences of secular knowledge, is by far the most blatant example of the great harm done to humanity. Even today, it's commonplace for Muslim brothers to murder their sisters who have been raped, out of some perverse sense of family honor, gays are put to death in Iran and elsewhere in the Muslim world, women are relegated to 2nd class citizenship at best and slavery at worse, girls are routinely pulled out of school at early ages lest they learn too much about the world around them (I saw this many times in the Muslim country I lived in for several years).