Master isn’t “going insane so now she sees monsters.” She was cursed 100 years ago so that the entire world seems monstrous, and has lived in isolation ever since. She met the MC, who is evidently exempt from the otherworld curses, and took him in since he’s the first person she’s been able to talk to for 100 years.
Loli Mistress is in the same boat, but reversed. Everyone sees her as a monster, so she has hidden away in an underground castle. She has the MC kidnapped to mess with another witch, but surprise he’s again immune to this curse. So she clings to the only contact she’s been able to have for a century.
Both of them are, understandably, a little crazy at this point. 100 years of frightened isolation will do that.
Then there’s the third girl. I don’t think it’s clear what her curse is yet. Maybe she’s not immortal like the others, which is why she’s hunting them and draining their magical energy. (I haven’t looked at the raws for spoilers.)