The ending explained

it seems everybody is hung up on the possibility of christina's ghost giving the young boy his sling back...because the professor falsely assumed the kid got his toy back the morning of her death.
however who actually told the professor that this was done AFTER her death? the kid only says she gave it to him, and he decided to use it when he knew for sure he could get away with one last "eff you" to the place he hated most. maybe christina gave it to him prior to the evening of the events and we were left in the dark about it.

so yeah, no supernatural stuff here folks.

all the "main" characters are "diaboliques" in their own way:
- christina for wanting to get revenge on her husband.
- Michel and his mistress for the plot they conceived.
- even the retired detective is evil in his self-righteous and holier-than-though mentality because he very well knew where the plot was heading and still didn't try to save christina; when he tells her she will be absolved of her crimes in the morning, it means he knew she was heading towards her own death, and thus the end of her miserable life. for if she survived the lovers, it won't be for long coz of her weak heart. it was the best possible way for her to go: swiftly, but also with one last torment of her deed (seeing her husband wake up in the tub).
as for the lovers,the are left to rot in prison, without any of the things they desired the most: each other, the money, the freedom of the "ruin" that was weighing them down so long.