I was interested in the manga at the beginning, but the final explanation for the game is so ludicrous - even if we accept that
an email can hypnotize people and make them able to psychosomatically decapitate themselves or cause themselves to spontaneously combust
(which we really shouldn't), this fails to explain things like the origin of the confirmation emails.
How did the virus magically know what they were doing? Why would the 'letters spelling out a secret message' ever be a thing? Why and how did the virus write a note about being in the classroom? Is the virus an artificial intelligence? Why did I not check the rating for this manga before reading it?
Besides the unbelievable plot, the characters are awful. Each one is portrayed as either a horrible person or a saint who's happy to sacrifice themselves for their friend(s), plus one sociopath who dies a horrible death with no apparent pain (but may be the least cringeworthy character nonetheless). There are also the adults, who rarely appear. The police have no interest in investigating the King's Game even though all the kids die in exactly the same way as the messages they receive, because that makes sense. The teacher is never in the classroom, and the kids have no concern about their parents or siblings at any stage, only the relationships they have with their classmates.
I disagree with the other reviewers who posted about the ending.
He definitely didn't defeat the King's Game, he won it by being the last person standing. Also the scene with his girlfriend is just a flashback IMO - it's meant to be ironic because the game asks him 'what do you want', and the only thing he wants is his girlfriend who just died because of the game. However, I wouldn't be surprised if this manga did throw in some random time travel just for the unbelievable hell of it, so I could be wrong.
Don't waste your time on this one.