Why? There is an explanation in the TV series Joy of Life that Xun Lin clearly knows that his sister Wan'er Lin likes Fan Xian and doesn't want him to get married.

The Assassination of Fan Xian was directed by Princess Li Chang. In his view, it is basically impossible for Fan Xian to truly master the financial power of "Neiku". Because Fan Xian's enemy is the future emperor-Prince.

After Xun Lin's death, Lin Xiang found Fan Xian, took him to the study and showed him Xun Lin's childhood calligraphy. Xun Lin's calligraphy was very good when he was eight or nine years old, and he had great ambitions, hoping to make great achievements in the future.

When Xun Lin was very young, he knew that his brother was a fool and his sister was a lung patient. As the only healthy person in the family, he is also a male, and he has a great responsibility to live for the whole family.

Fan Xian didn't appear, take refuge in the prince and princess royal. Later, the Qing emperor ordered Wan'er Lin to marry Fan Xian, and things changed, but Xun Lin didn't know Fan Xian's real life.

In Xun Lin's eyes, Fan Xian is just the illegitimate child of the minister. Even if he is a little literary talent, he has no chance to win the financial power of Neiku. On the one hand, it is the illegitimate child of the official, on the other hand, it is the emperor's own son and princess royal. Xun Lin has no choice in this matter.

Xun Lin believes that Fan Xian will definitely become a political victim in the future. If he becomes his brother-in-law, not only will his sister be widowed in the future, but the whole family may also be implicated by Fan Xian. So he decided to assassinate him. This is not because Xunlin is too bad, but because he takes the Lins very seriously. He can do everything he can for the Lins.

Blame it on Emperor Di Qing. God's grace is unpredictable Who can know in advance what he thinks of Fan Xian? Everyone knows that Fan Xian is his blood.