Why did Vader in Three-body kill Cheng Xin?

Cheng Xin was an orphan and was later adopted by Cheng Xiuxiu. Later, Cheng Xiuxiu's boyfriend broke up with her because of Cheng Xin. In the crisis era, he was a doctor majoring in aerospace engines. After graduation, he joined the research team to develop engines for a new generation of Long March rockets, and was later transferred to the Strategic Service Office of the Planetary Defense Committee.

Put forward the ladder plan at a meeting and become one of the participants of the ladder plan. The ladder plan was introduced when Vader was introduced earlier, so I won't go into details here. Cheap occurred because the detector of the ladder plan did not enter the predetermined orbit. Cheng Xinran went into hibernation.

In the era of deterrence, he ran for the second swordsman, but when he took over, he did not launch the broadcast, but was launched by a three-person group. As a result, humans began to immigrate to Australia, and the earth security forces and resistance organizations came into being. Cheng Xin also blames himself for being sick and blind.

In the later Bunker era, Star Ring Company entrusted Vader to develop a curvature-driven spacecraft. Because of Vader's promise, and the solar system alliance thought it was not in line with federal law, Vader kept his promise to Cheng Xin and gave up his life.