Why does the Shafi'i Madhab permit killing all non Muslim adult males during Jihad? Is it just because of their disbelief? Isn't that too cruel?
Ibn Rushd writes:
They disagreed about the case of hermits cut off from the world, the blind, the chronically ill, the old who cannot fight, the idiot, and the peasants and serfs. ... According to al-Shafi'fs most authentic opinion, all of these categories (of people) are to be put to death.
The Distinguished Jurist's Primer Vol. 1 p. 458 translated by Imran Ahsan Khan Nyazee
Ibn Rushd writes elswhere:
The basis for their disagreement stems from the conflict of the specificity in some traditions with the general implication of (some verses of) the Qur'an, and also the generality of the authentic saying of the Prophet (God's peace and blessings be upon him), "I have been commanded to fight mankind until they say, 'There is no God but Allah.'" The words of the Exalted, "Then, when the sacred months have passed, slay the idolaters wherever ye find them", imply the slaying of every nonbeliever whether or not he is a monk, and so does the saying of the Prophet (God's peace and blessings be upon him), "I have been commanded to fight mankind until they say, 'There is no God but Allah'".
The Distinguished Jurist's Primer Vol. 1 p. 458-459 translated by Imran Ahsan Khan Nyazee
A woman, minor, enfeebled old man, an invalid, and a blind man are not to be killed. ... Al-Shafi'i (God bless him) goes against our opinion in the case of the enfeebled old man, the invalid, and the blind. The reason is that permitted killing in his view is based upon unbelief
Al-Hidayah Vol. 2 p. 293 translated by Imran Ahsan Khan Nyazee
Reliance of the Traveller
It is permissible to kill old men ... and monks.
Reliance of the Traveller o9.10 translated by Nuh Ha Mim Keller
Follow Up Question
Does this mean that not all non combatants are impermissible to kill?