The demon (species), imp, and the demon lord and her followers, are themselves described as being extremist, acting in the interest of turning literally every human woman into a monster. There's plenty of monster neutral territories that welcome monster/human relationships without trying to convert everyone. And then there's the order, which is implied to be using deception and religious extremism to make people believe monsters are evil.
KC has said in some interview that there's basically no way for this world to end in any sort of consequences because as soon as every human woman is a monster, there will be enough accumulated demonic energy to give the demon lord enough power to make them capable of giving birth to human males. So any concern one might have over whether monsters could ever cause the extinction of humans is entirely moot.
The particular line of grim and less optimistic fantasy that involves the potential on-screen death of monsters is not really of interest to me, and I think there's some rules on this particular site for writing stories that are too detailed in the level of violence portrayed against monsters. I don't know what the consequences of writing one look like.
If you do publish them, and find people aren't receptive, or even someone tells you that you have to take them down, try putting them on touchflyffytail, which has a larger audience and a greater license for how you write monster girls.