Why did Shiro, the Babylonian star in Tokyo, kill Subaru Sumeragi's sister?

The duty of the Sakurazuka family is to kill everyone in the royal family. In order to prevent Shi Lang from killing Subaru Sumeragi, Hokuto Sumeragi (Subaru Sumeragi's sister) pretended to be Subaru Sumeragi and was killed by Shi Lang. Star White Shadow passed through Hokuto Sumeragi's heart by hand. Hokuto Sumeragi hoped that both Star White Wolf and Subaru Sumeragi could survive, so he cast a spell on the dead Star White Wolf, that is, if Star White Wolf killed Subaru Sumeragi by killing Hokuto Sumeragi, his method would bite him. But after the star Shi Lang had feelings for Subaru Sumeragi, he didn't want to kill him. On the contrary, he hoped that Liu Zan would commit suicide. Therefore, the star Shiro cast a spell on Subaru Sumeragi that was forbidden by Hokuto Sumeragi. As a result, Kojiro died. Subaru Sumeragi was very sad, while Subaru Sumeragi hoped that the Star White Wolf would kill him!

Star Shiro confessed to Liu Zan before he died, but he fell down halfway.

"I ... you ... I support you ...''