Question:  In times of war, are there any rules of engagement for a mujahid? Is there a punishment for him if he slays an innocent or an unarmed person especially old men, women and children? However if such people are aiding the enemy, is it still forbidden to kill them?
    Answer: During warfare, Islam has prescribed certain ethical principles that every believing combatant must comply with. These principals entail that old men, women and children not be killed. Also, trees must not be burnt and standing crops should not be destroyed. As Anas Ibn Malik narrates:

The Prophet (sws) said: Go in Allah’s name, trusting in Allah, and adhering to the religion of Allah’s apostle. Do not kill a decrepit old man, a young infant, a child or a woman. Do not be dishonest about booty but collect your spoils, do right and act well, for Allah loves those who act well. (Abu Da’ud, No: 2608)

    However, if a warrior is guilty of overruling any of these principles, he will be dealt with according to the nature of the crime that he has committed. Thus, if he kills an innocent person of the enemy country deliberately, he will stand susceptible to the punishment prescribed for the intentional murder i.e. Qisas or payment of Diyat if the plaintiff chooses to accept it (2:178-9). And if the victim is a believer of the enemy country and has been murdered unintentionally, the murderer will have to fast for two consecutive months (4:92-3). Both types of cases shall be considered in a proper court of law in the country for whom the criminal has been fighting and no individual has the authority to take the law into his own hands. It is worthwhile to mention that the Shari‘ah has not prescribed any punishment for the case where a non-Muslim of the enemy country is killed unintentionally. Thus, this matter and other crimes, for instance vandalism, shall be dealt with according to what the government or the international human rights bodies legislate in this regard. As an obvious fact, a combatant should not be held responsible for killing an old man or woman who was actively taking part in the war.