Complex sentence: Rick stares at the ground in shock and is approached by Negan, who bends down to Rick's level and begins to taunt him, but Rick calmly mumbles that he is going to kill Negan.
Split into simpler sentences: Rick stares at the ground in shock and is approached by Negan. Negan begins to taunt Rick, who calmly mumbles that he is going to kill Negan.