First, because she wants Wan'er Lin to rely on better in the future, after all, Wan'er Lin is her own daughter, because if Wan'er Lin marries Fan Xian, then Neiku will be in Fan Xian's hands. As a man who holds the wealth of a country, he must have suffered many assassinations. Now the royal family is in a chaotic period. Maybe Fan Xian will be killed one day. To avoid this, she will kill Fan Xian first. So Wan'er Lin won't marry him. She thinks Wan'er Lin will have a better home. Second, because princess royal may collude with others and think that Fan Xian is too clever. If Fan Xian continues to develop like this, then one day her crimes will be reported and she will have a bad ending, so she thinks it is necessary to kill Fan Xian. Generally speaking, princess royal wanted to kill Fan Xian because some interests of princess royal were influenced by Fan Xian.