Answer: The tone is accusing.
In this excerpt, Creon is talking to Ismene in an accusing way. He calls her snake, implying that she is sneaky and deceitful. He also accuses her of "sucking out my life's blood" and calls her a "pest." The words are harsh, as Creon believes that Ismene wants to rise against his throne, and therefore, is plotting something that will lead to his demise.