Why didn't Darth Vader kill Third Sister?

[Bumble]

In Obi-Wan Kenobi, Third Sister worked for Darth Vader under the pretense to get very close to him and kill him. She even killed The Grand Inquisitor to take his place.

Darth Vader knew all these and when Third Sister finally made the attempt to kill kill, he beat her well but just stabbed her in a way not to kill her.

Why did he not kill her?

 

[Dori]

She wanted her to suffer knowing well that she failed. What I don't understand was why he kept using her knowing her plans all along?