In a recent interview, the fantasy author George R. R. Martin defended his practice of killing off popular characters by stating that war affects everyone, not just minor background characters. However, this authorial decision has resulted in a dwindling number of key players in the series, which just shows how fond Martin seems to be of murdering his characters.
As a writer, George R. R. Martin has a responsibility to his readers to provide compelling and suspenseful content. By consistently killing off characters in the "Game of Thrones" series, he keeps readers intrigued and interested in what is coming next. When characters are killed off in the series, it creates new landscapes within the story that his fans have come to know, love, and expect.
No, George R. R. Martin has killed off as many characters as he feels is necessary. As an author, he has a right to decide the fate of every character. If he believes that the death of a character contributes to the plot of the story, then the death is necessary no matter how many have already occurred.
George RR Martin does not kill off people in his books and in the television series simply because he can. It is part of the larger story that he is weaving. The fact that no one is safe is a part of the theme that greed and desire cause big ripples and grief.