He's actually taking this seriously. How so? Naruto is still alive.. If Naruto was slacking, he would be by long dead.
Come on, you know it's hard for Naruto. Even if he says I want to kill you, it won't be easy to kill Sasuke. Just like what Haku said to both Naruto and Sasuke, and what Kakashi said to Sasuke that he is going to finish him for the village's own good. In the end, Kakashi was overwhelmed.