About the ending (spoiler!)
The boy says Christina gave him the sling, but he doesn't say when - maybe it was before her alleged death. Michel was checking her pulse and didn't he conclude she was dead? How did she fake that, at that time (no medicines available like in Saw, for example)?
The ending is left open for interpreting on purpose. Maybe the boy really saw her and she isn't dead, maybe he saw her ghost, maybe he IS lying this time (like many times before) and she is dead cause the detective got there too late. Some movies are simply more beautiful when they intrigue you even when they're over.