For me the case that scares me the most is that of Diao Aiqing
She was an introverted computer science student at a good university in China. Not known for going out, no close friendships or anything much outside of her studies and as she came from a rural area she didn't know anyone at her university particularly well. Basically just someone who kept herself to herself and worked hard.
She was killed brutally, I don't even want to list what was done to her but it was horrendous and parts of her body have never been found. It was impossible to even tell how she died.
There has never been any possible motive put forward officially, never any suspect and basically no evidence.
It honestly breaks my heart and I just can't comprehend how someone could do that to a human being once but seemingly never again. Most murders happen either because it is personal and the killer feels 'justified' , its to do with the killier wanting something (money etc), it's part of a spree or serial crime, or its happened due to the killer's mental illness.
This is a case that doesn't seem to fit with any of those things as she had no close contacts around her, was from a poor family, there were never any similar crimes reported, and it seemed so organised.
I lived in China for years and a friend of mine who studied at Nanjing said their were certain rumours but I sadly don't think it will ever be solved.