Why did the Trisolarans announce their one weakness?

Question

Early in the novel, Ye Wenjie makes a brief appearance and suggests to astronomer Luo Ji that he consider creating a new branch of science, the sociology of interplanetary society. This would be based on a few axioms: life expands infinitely but matter in the universe is limited, the potential for technological explosion at any time, and what she calls "chains of suspicion". It's a very dark, zero-sum speculation about the relationships that might potentially exist between technological civilizations on different planets. Ye Wenjie was a physicist who created the Trisolaran crisis by signalling to the Trisolarans and inviting them to invade. It seems she is now making this suggestion out of a sense of regret and possibly considers it her duty to pass on this idea.

Luo Ji is chosen to be part of the Wallfacer project. This is an attempt to work around the Trisolaran's ability to monitor human activity and communications: since they can't read minds, the Wallfacers will create deceptive strategies known only to themselves. Luo Ji is a second-rate academic who does not embrace his awesome responsibility as a Wallfacer. Instead he uses it as a chance to find the girl of his dreams and create a comfortable lifestyle for himself. It becomes clear that the only reason he was chosen as a Wallfacer was because the Trisolarans had ordered assasination attempts against him, therefore they evidently considered him a threat somehow. And later, Luo Ji realises that this had to do with his conversation with Wenjie earlier.

However, the Trisolarans and their human accomplices knew or ought to have known that Luo Ji didn't understand the significance of what he was told. By trying to kill him, they made known to humanity that Luo Ji was somehow a greater threat than the other measures being taken. It seems that it would have been more prudent for them to simply keep him under observation. They could even have supplied him with women and other distractions to make it more likely he would simply forget about his conversation with Wenjie, but if he did appear to be about to act on the information, they would still be able to deal with him.

Why did they give the game away by trying to kill him?

Answer 1

The Trisolarans were too unsophisticated, and the ETO didn't know the context

It's explained early in the Dark Forest that the Trisolarans communicate using a form of telepathy, hence their minds are transparent to each other and their behaviours are relatively straightforward. They don't understand deception or indirect behaviour.

"But transparent thought has become a cultural and social custom. This might be hard for you to understand, just like it's hard for us to understand you." "I can't imagine that deceit and scheming are totally absent in your world."

Therefore it didn't occur to them that not killing Luo Ji (and hence not drawing attention to him) might be more prudent.

The ETO members understood Trisolaran psychology and helped to compensate for its weaknesses where they could:

All of the skills that you lack - deceit, trickery, disguise, and misdirection - we use in your service.

However they were not told the reason why Luo Ji needed to be eliminated. If they had known, they might have pointed out to "the Lord" that in this situation, they could have safely left Luo Ji alone to amuse himself while still keeping him under observation in case he showed signs of understanding the significance of what he had been told.

Answer 2

While not baldly stated in the story, it's doubtful that Ye Wenjie mentioned her fears to JUST Luo Ji. Or indeed that Ye was the only person to think about interplanetary interaction in the same way. After all, there are dozens of scifi books premised on similar actions. The Trisolarians and their human minions may have been on a worldwide crusade to eliminate anyone who thought deeply about such things and perhaps anyone (or maybe just close associates) they specifically told the idea to. If so, Luo Ji doesn't become "the one guy they tried to off" and instead is "the chance lone survivor of hundreds of assassinations that went off with nobody the wiser."