Why didn't the Witch kill or capture the Emperor already?

Question

In Mulan (2020), Xian Lang's character is a sorcerer of some sort addressed as Witch throughout the movie. Right from the beginning she is shown to have capabilities to impersonate others/possess their bodies to infiltrate.

In two instances she has successfully infiltrated the palace and is in close proximity of the Emperor.

But even then she does not try to capture or kill him. Why is that so?

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Edit

As answered by Yu Zhang, Boris Khan wants personal revenge which is why the Witch does not kill The Emperor.

But my point is, it's due to the Witch's help that Boris Khan is able to reach the Emperor and take him as his prisoner. That could have been done easily by the Witch. Why go to war? Kind of a long way to get to the Emperor. That seems like a plot hole to me, hence the question.

Answer 1

The Rouran leader Boris Khan wants a personal revenge, as the Emperor has killed his father, therefore Boris Khan wants to kill the Emperor personally and he wants a public execution so that everyone will see.

The Witch is not portrayed as a bad person but rather as a misunderstood person, she redeems herself in the end in front of Mulan. Having the witch kill the Emperor will make this redemption less credible / likeable.

Besides, plot-wise, if the Emperor has been killed, how do we end this movie?

Answer 2

Boris Khan wants revenge and also wants to become the king. That was their initial plan. The witch joined forces with Boris because she believed he would make a difference to people like her when he took control of the kingdom.

If you kill the king by abducting him, you won't become the ruler. You have to conquer him. Otherwise, the next royal blood will become the next ruler.