I'll be honest, right from the start this rather struggles. The premise could be interesting. The first fifth of the picture to greet us suggests that it could quite reasonably have proven to be a low-budget, understated psychological thriller, confined to a single space, to great success. Unfortunately, the most promising ideas conflict with the heightened sensibilities about the movie's construction, and that's to say nothing of the ways in which it plainly falters. Sadly, 'Sinful' fails to inspire.
In and of itself Toshiyuki Hiraoka's music is decent, if uneven, serving up pervasive soundscapes that lend an air of tension. That factor would be more meaningful if it were less prominent in the audio mix - and if the movie were more nuanced. However, the scene writing, characters, and sound effects immediately come across as blunt, frankly over the top, and therefore annoying. Whether stars Nicole D'Angela and Christina Lo are to blame or filmmaker Rich Mallery I don't know (I tend to give actors the benefit of the doubt, as they can only work with what they are given), but the performances drawn out here likewise seem entirely too forthright and unsubtle for the piece's best interests, if not also inauthentic. Dialogue is flagrantly unconvincing and sometimes repetitive, somewhat pulling one out of the viewing experience. Useful to a small extent to provide background information, the cuts to shots and scenes outside the active narrative are overused.
While it establishes that characters Salem and Remy are romantically involved, 'Sinful' doesn't do a good job of giving this facet any importance in the story. In one place or another we've all seen the tired representation of two women passionately kissing and spending all their time together as merely "gal pals," and the screenplay rather seems to treat the relationship here just as lightly, with no more seriousness. Plot development feels scattered and carries no real weight - a pity, because it definitely should. The movie tries to build a sense of mystery, and cements strained dynamics between the characters in the scenario, but as it presents the narrative is less than believable at any point.
I really do think there are some good ideas here, but this is a concept that required a substantially more delicate hand, a far more mindful approach. What we get is instead immensely heavy-handed, heedless, and kind of outright flimsy - and predictable. Even noting poor reception I sat to watch with hope in my heart, but that hope was quickly dashed. There are countless other movies you could watch instead; you don't need to spend time with 'Sinful.'