Well if I put a hungry lion in yout bedroom then locked the door after you went in I think that might be considered murder assuming the lion killed you So yes it is not the instrument but the person arranging events who is the murderer I recall that he did control the basilisk on acount of being able to speak to it and release it at will He later sets it loose again on Harry In context it might still be correct to say she was killed by the basilisk but also murdered by Voldemort