Hajime Nagumo, casually killed in a moment a hundred thousand demons (which basically are brown humans with good magic), killed several billons of cockroaches with superpowers (and was revealed in the spin off that they had human level intelligence), and some billions of monsters. The first two feats happened in his 7-B key while the last one was in the tier 6 key.
Mother for Low 6-B since she annihilated the humanity of a world more developed than Earth, which didn't faced wars and natural disasters that reduced their population, that basically cured hunger and didn't suffered from terrible diseases do to their technologycal level, and that even had settlements in other planets. She also killed all the other AI in existence which should count since they had emotions and all.
Also,
Ehitorujue is High 6-B so change him to there. His kill count is more severe than what the description make it look because in the spin off we were show how he took hostage the entire humanity against the Liberators and were making them killing between themselves (parents killed their sons, childs killed their brothers and parents, lovers happily killed their partner and things like that all over the world) just to torture the Liberators, this isn't even counting all the other wars that he caused, species that make extinct, and ages that destroyed.