Why did the chief priests make plans to kill Lazarus as well as Jesus?

Answer: Many believed in Jesus because of him

Crowds came to see the raised Lazarus, and on account of him many Jews were believing in Jesus.

“Meanwhile a large crowd of Jews learned that Jesus was there. And they came not only because of Him, but also to see Lazarus, whom He had raised from the dead. So the chief priests made plans to kill Lazarus as well, for on account of him many of the Jews were deserting them and believing in Jesus.”— John 12:9-11 (BSB)