User blog comment:B214/Question and Answers 6/@comment-37402804-20181104095558/@comment-37165251-20181104103006

"How Could You Possible Kill God" ... a god Question.

But in the DxD Canon God was allready severly weakened because he sealed Trihexa with thousand Seals (partly forbidden) away and had no chance to recover his Power before the Start of the Great War. Given that he also had to fight to contain two Heavenly Dragons before his Final Fight against the four Original Maou it is good possible that even he could die.

That Rias let Issey Die is a loved Part in many FanFichtions, but I think not she did it (and as far as I know it was not the case)

Rias is still young and a sometimes naiv. She thought the fallen Angels wanted to recruit Issey (and given that they are all seductive and Issey is a Pervert that was highly possible). On the other Side if she let him die and he ever realized this how would he react....or Ddraig. To have a enraged Dragon as an Enemy was something even Rivezim found not to be good (and that was only a Dragonking). Issey furious and overwhelmed by Sadness going into Juggernaut was definetely not something Rias could have wanted, and she didn´t know what sacred Gear he had. That she became first aware in the Church as he beat Raynare.