I also don't buy into the whole 'Maya becomes a drug addict/prostitute' thing either... I just think it's not
unlikely that either something very bad is going to happen to
someone over the course of the story or we'll find out something very tragic about their past. Obviously, the most likely candidate for this is Bella, considering the locked room, her closed-off demeanor, and the MC's line in the library about 'if I knew then what I know now, I would have screamed for help' (paraphrasing, since I don't recall the exact line).
My wild-ass theory (which I'm sure I'll get mocked for) is that the locked room is a nursery. My crazy guess is that Bella and her husband suffered the loss of their child either by late-term miscarriage/stillbirth or infant death, the stress and shock of which broke their marriage (something I unfortunately have seen happen to two different couples personally). The husband left and Bella had a breakdown of sorts that has prevented her from accepting that her husband isn't coming back. This would be sufficiently 'dark' to warrant the disclaimer, but doesn't necessarily fit the library line completely (I never said it was perfect

). The obvious 'Bella killed her husband and his remains are in the room' thing is
too obvious and the axe is likely a red herring.