The version I heard was that the fifth hitman contacted the victim-to-be, who agreed to fake his own death. That plan was foiled, leading to everyone in the chain getting caught. I have no idea if this is actually true, though; I think my source may have been a tumblr post.
From the article, which apparently here actually no one read:
> However, hitman number five was so incensed at how much the value of the contract had fallen, that he told the target to fake his own death, which eventually led to the police finding out about the plot, Beijing News reported.
Combined with the quote you posted in response to my comment, this does make the article (which didn't load for me) seem confusing. One one hand the fifth hitman 'eventually told the police', but on the other hand 'he told the target to fake his own death, which eventually led to the police finding out about the plot'. On balance it seems like hitman 5 didn't initiate contact with the police, but confessed when caught.
Also I wonder why the 5th hitman sentence is almost as harsh as 2–4th ones.