Is it permissible to shed the blood of a person who impersonated a girl to speak with the questioner's sister, attempting to discuss immoral matters with her, and if not permissible, what is the ruling of Islamic law concerning it?

It is not permissible to kill a Muslim except for one of three reasons: the married adulterer, a life for a life, and one who abandons his religion and separates from the community. The execution of the penalty is for the ruler or his representative, such as a judge. The sister should avoid speaking with men, and the man should refrain from speaking with the questioner's sister and repent.