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?