First off, I feel like the synopsis for this manga is painfully inaccurate. The first 4 or 5 chapters touch on the themes of murderous impulses, but after that it's a fluffy school romance story and definitely not a "fearsome psycho-thriller". However, I didn't really mind this switch in themes because I'm personally weak for sweet school romances, but if you wanted a thriller examining the complexities of adolescent feelings then this isn't for you.
Story - 5
There's not much to say. The story involves Ichikawa, a loner at school, and his crush on Yamada, a model who has a sweet tooth. There's nothing particularly revolutionary about the story, but it doesn't fail in any areas. It's perfectly average, no more and no less. This hasn't been completed yet so the story could get really bad or really good, but in its current state it's a simple yet vaguely pleasant read.
Art - 7
The art is good. I can't say much more about it. It's appealing, but it doesn't try to break from the mold.
Character - 6
Once again, I don't have much to say. The characters aren't incredibly deep, but they aren't annoying and break past tropes enough for them not to feel entirely stale. I did like how the protag's ignorance of Yamada's affections towards him actually seemed realistic and his thought process makes sense, although I didn't really like how the series dropped his murderous impulses after a few chapters.
Enjoyment - 6
It's not spectacular, but it's definitely not bad. I didn't enjoy it as much as I enjoy other manga I hold dear, but I never found myself wanting to close it. It's solidly average and a good manga to read in between longer and more action-packed series.
Overall - 5
I felt like Boku no Kokoro no Yabai Yatsu is average, and that's not a bad thing. If you feel like the description of a sweet middle school romance between an outgoing girl and a shy boy has you hooked, then I recommend you read it. If not, search elsewhere.