Why was the Father of the Archons left alive and imprisoned?

But if the Sons of Darkness were skinned and stretched in the air, they give evidence that Darkness, their Father, is also mortal because he is composite. Why, therefore, did they not skin him, too, in the beginning and deliver creation from his injuries ? What necessity could there be that he should be left alive, and what reason 3 was there in his case that he should remain and turn again to struggle with pure souls ? . . .  [Cf. p. lxxiii.] And after he has 'intoxicated' and perverted and put them to shame, after he has made some of them fornicators and minstrels and blasphemers, then that wise Builder and Architect [Cf. p. xxx. l. 14.] has sense enough to frame a Grave and Prison for him. And instead of the Prison-house being thus built after a long period, and with [P. 15.] much toil, if the Sons of Light had been gathered together and with these Stones had stoned him, then, lo, [Cf. p.xxi. l. 32.] he would have come to an end. But if he would not have died, because his nature is not mortal, then this impure Teaching is put to shame in everything it says. For how did the sons of the immortal die, and how were the sons of a spiritual one skinned, and how were those [xxxvi] who are not composite disintegrated ? [Mani himself was skinned.] And they did well who skinned the lying Mani, who said that Darkness was skinned, though it has neither hide nor Sheath-skin.