Why didn't Li Shimin kill Wu Zetian to avoid future trouble?

Li Shimin didn't kill Wu Zetian, not because he loved her too much, nor because the warlock said that killing her wouldn't change the short-lived usurpation fate of the Tang Dynasty, but because Wu Zetian didn't show any threat to the Tang Dynasty during the Li Shimin period. She is a woman, her father is not a powerful person, and she has no family power. At the same time, Li Shimin never gave her any prominent position. How can such a person become a threat to the country? Since there is no threat, there is no need to kill people. Yuan Tiangang's prediction is that Wu Zetian created public opinion for her usurpation. There is a certain chance that Wu Zetian can be emperor. Tang Gaozong liked her and made her queen. At the same time, Emperor Gaozong was in poor health, and sometimes she was asked to handle government affairs. She won the trust of some people by handling government affairs. Besides, her sons are basically not people who can do great things. The Tang Dynasty was a relatively open dynasty, which also laid some ideological foundation for her to be an emperor.