@Sancturil Well, the story is told from Rimurus point of view, so of course we think about the humans and the church as the "evil" ones here, but while SOME of them are (Like the king who just wants more power, its not like he really attacks because its monsters, but because he can justify it and gain much from it if he wins or maybe some world travellers, but thats debatable, the boy feels evil, but isnt it more the stealing of his live and bringing him into slavery, using him as they whish? Would spark mental problems with most people i think, the brutal behaviour maybe just the way to deal with it after he snapped, who knows ^^), not all of them are. Like, the soldiers and stuff, i mean, the boy killed last in the chapter? What was he? 17? 18? Some boy in military service, thats the ones that feel the failures of their commanders and rulers after all, without fault of themselves.
The situation itself is partly new, monsters that maintain diplomatic relationships and stuff and as bad as that seems, they way Rimuru wants to do it seems the most reasonable now, killing as much as he needs and becoming a demon lord establishes them as a force instead of a target and that is needed. Thats shit for the ten thousands that have to die for it, but its better than letting hundreds of thousands in the city die/become slaves to the human kingdom.
After they have been recognized they can try to establish slowly a more peaceful way and maybe even become sort of a mediator between the demon lords countries and the humans.
