The killing of innocent civilians or non-combatants is not allowed. Even in a state of legitimate war, it is prohibited to attack old people, women, children, hermits, businessmen, disabled people, and the ill. It does not matter what religion they are.
What the Qur’an and Sunnah Say
Allah says, “Take not life, which Allah has made sacred, except by way of justice and law: thus He commands you, that you may learn wisdom.” Qur’an, Al-An’am, 6:151)
Allah says, “For that cause, We decreed for the Children of Israel that whosoever kills a human for other than murder or corruption in the earth, it shall be as if he had killed all humanity; and whoso saves the life of one, it shall be as if he had saved the life of all humanity.” (Qur’an, Al-Ma’idah, 5:32)
Allah says, “O you who believe! Stand out firmly for Allah, as witnesses to fair dealing, and let not the hatred of others to you make you swerve to wrong and depart from justice. Be just: that is next to piety: and fear Allah. For Allah is well-acquainted with all that you do.” (Qur’an, Al-Ma’idah, 5:8)
It was reported: “A woman was found to have been killed in one of the battles, so the Messenger of Allah announced the prohibition of killing women and children.” (Bukhari #3014)
The Messenger of Allah, when sending off an army, would say: “Leave in the name of Allah, but do not kill an old man, nor a young child, or a woman…” (Abu Dawud #2614)
Some people argue that it is allowed to attack the citizens of a country whose army is killing other innocent people, such as the United States wars in Afghanistan and Iraq. Such reasoning is incorrect. Islam does not condone responding to a wrong deed with another wrong deed. If you disagree with the foreign policy of a nation, you must use lawful means of trying to stop them from doing wrong.
Shaykh Mustafa Umar