Why does hollyleaf kill firestar?

What do you mean? Hollyleaf doesn't kill Firestar...

I guess you may have been thinking of Ashfur...she would never hurt firestar(Rosalinaluma72)

he was her clan leader!And she was bound by warrior code so she could NOT do that.

And if your asking why she killed Ashfur is because she didn't want him to destroy her an her littermates at the gathering with that secret.(Runningbreath)