Dude brought this upon himself. He's the one who keeps ordering her to withstand the consequences of her mana abnormality for days, despite knowing it will accumulate and accumulate lust endlessly. It's like you took an obese person and told them to keep eating zero calories, tasteless soup everyday, yet when you daily meet the person, you'd yourself eat croissants and pizza in front them.
Mc is being mean to a girl who has nothing to do with everything happened omg
As far as we have seen, this is a series where demons and humans are incompatible, fundamentally incapable of living side by side. The demon queen was so far off she couldn't even live among the other demons, apart from Archive, just barely. So, in reality, the MC wants to kill Idly, but he needs her to kill her father, the hero. We can assume Archive is somewhat "polluted" by occupying a human body, plus he was the weakest demon, which probably explains why he can keep living how he does, among humans. Still, we have seen he's really murderous in reality, which leads to the carelessness he sometimes displays.