I don´t know how it is today, but in the past every consigliere was elected to his position by the members. Killing him without the captains accordance would be undermining the boss´s position and jeopardize the boss´s relationship with the rest of the members. The Consigliere spot was a position of confidence. That´s why the consigliere position used to be an extremely powerful one. Mostly, this rule had to do with securing the boss´s own position because he was also supposed to get elected to his position but it was also a tool to keep the power balance between the boss and the rest of the members.