It is pretty hard to see a solitary isekai MC, like Kumoko, becoming the underling of a destructive demon lord. Usually in isekai if the MC becomes a demon lord's underling, the demon lord is either not evil, is misunderstood, is a crybaby, etc. I can't currently imagine Kumoko going around slaugtering humans wholesale. Who knows what the stated methods for preventing the world's destruction are, but wouldn't people rather think an isekai MC would find some novel way to do it if the official way is outrageous?
Maybe I'm just too soft, haha.