Sahih Muslim 33—The Messenger of Allah said: I have been commanded to fight against people till they testify that there is no god but Allah, that Muhammad is the messenger of Allah, and they establish prayer, and pay Zakat.
The Qur'an commands Muslims to fight the unbelievers, and to be severe against unbelievers (but merciful among fellow Muslims).
Qur’an 9:29—Fight those who believe not in Allah nor the Last Day, nor hold that forbidden which hath been forbidden by Allah and His Messenger, nor acknowledge the Religion of Truth, from among the People of the Book, until they pay the Jizyah with willing submission, and feel themselves subdued.
Qur’an 9:73—O Prophet! strive hard against the unbelievers and the hypocrites and be unyielding to them; and their abode is hell, and evil is the destination.
Qur’an 9:123—O you who believe! fight those of the unbelievers who are near to you and let them find in you hardness.
Qur’an 48:29—Muhammad is the Messenger of Allah, and those who are with him are severe against disbelievers, and merciful among themselves.
Qur’an 9:111—Surely Allah has bought of the believers their persons and their property for this, that they shall have the garden; they fight in Allah's way, so they slay and are slain.
This is why no one is ever surprised when Muslims kill non-Muslims.
BAGHDAD (AP) -- Iraqi police are reporting a new attack on Christians in Baghdad by gunmen who broke into the home of an elderly couple and killed them.
It was the latest in a series of attacks on the country's Christian minority, which has been fleeing the country in droves since an Oct. 31 assault on a Catholic church that killed 68.
Al-Qaeda's front group in Iraq has threatened Christians, saying the violence is retribution for Egypt's Coptic Church holding women captive for converting to Islam.
Police said four gunmen raided the home in a predominantly Shiite neighborhood Sunday evening and repeatedly shot the couple with silenced pistols before escaping. The officials spoke on condition of anonymity because they were not authorized to speak to media. (Source)