A few thoughts.
It bears repeating that people here are absolutely appalled by this crime. In no way does anyone consider this acceptable.
It’s also my experience that there is very little help for the mentally ill.
The general attitude is that there are many people and limited resources in China, so those resources ought to go to the people best able to contribute to society. What this means, in practical terms, is that even fairly mild disabilities can prevent you from going to college, getting a decent job, etc. People who we would consider pretty much functional are often warehoused in special homes with no chance of getting out. Parents of people who commit suicide will often refuse to claim the bodies- a lake at my friend’s university has thirteen unclaimed bodies, a number that grows each year.
Since the stigma is that big and that life-changing, people who need help are extremely reluctant to seek out whatever small help there is. I have a friend who had mild depression, and he got it treated in a town four hours away, because he was so terrified that someone would find out.
I think China may be especially prone to copycat crimes for a couple of reasons. One is that everyday crime is rarely reported. The news is mostly feel-good human interest stories and fairly hands-off stuff. I’m told there is a law that unsolved cases cannot be reported on, which means a lot of missing person stories and the like never make it on the news. There really isn’t a lot of news on everyday murders, etc. So a story like this would stand out even more in people’s heads- even the heads of psychos.
Secondly, it’s a strongly conformist society. I wouldn’t be surprised if even the crazy people choose to take the well-trod path rather than show individuality. If there gets to be a societal script that “crazy people stab children,” then when someone starts feeling a bit crazy, they may look towards that script first.
Childhood does have some different connotations in China, which I don’t know enough to comment on too much except that few of my students say they had a happy childhood. From my impression, it can be a stressful time. School lasts a lot longer and there is strong pressure even from elementary school. Weekends and time off is filled with cram schools or prestige arts (piano, classical dancing.) In rural areas, the parents often work elsewhere and children are left with their grandparents or other relatives, which in my experience is really difficult for the children to handle and makes them feel lonely and unloved. Anyway, I don’t know if this is a factor, but it may be that some of these people have deep resentments that stem from childhood.
Finally, the explanation (“I was disappointed with my romantic life and society”) may seem a bit less callous in Chinese. China tends to look for outside explanations for people’s behavior, rather than internal ones. What is a reasonable explanation to Chinese people often seems like a lame excuse to Western people.