Ana Satsujin AKA Peephole sits in a very specific niche as far as mystery/drama/horror manga are concerned. It's not very horrifying, although it is gory and violent. It is rather dramatic, although more so in that it provides schlock and easily-predicted plot twists by the handful. As far as mystery goes, there isn't much to be found past the first arc, even if you're particularly naive and can't pick up on the generic tropes presented in the "trash gore-romance drama" genre of manga.
However, this doesn't mean that Ana Satsujin isn't an enjoyable read; Supposing you're a fan of this type of manga or looking for a little bit of garbage in your gore-stew. I myself enjoy schlock, and this manga embodies that entirely: Part Fatal Attraction, part generic serial killer manga, Ana Satsujin satisfies a specific itch that many B-grade manga approach but few actually are able to scratch. It's a shitty thrill ride that is well worth the, admittedly cheap, price of admission.
The characters are one-note, the plot points are contrived, and the twists are so on the nose you'll call them long before you see them; but the point stands, it's just so dumb and goofy that it's hard to put down. Consider it more a horror-comedy/black comedy and chances are you'll laugh and cringe your way to the end before you know it.
It won't blow you away, it's not a masterwork, or even particularly good.
But I'll be damned if it isn't "So bad it's good-ish."