Nate is a character we learned who is a lone wolf and crazy. We learned that he went to the Gil's station with a mask and tried robbing the old couple. Nate left and found his friend killed by Eddie and Wyatt. Nate chased them down. After Eddie or Wyatt got away Nate met Russel. Nate went back to the Gil's station with Rus and killed the old people. Russel left Nate on his own.
Now here's where the theory starts. After Nate left the Gil's station he could have been searching the woods for Eddie and Wyatt. Nate failed to find them in the woods and came across Katjaa and Duck in the woods. He threatened to kill Kajaa if she didn't tell him where Eddie and Wyatt were. Katjaa didn't know so he killed her and left. He shot her like he did to the cop in 400 days. Duck looked dead to him already so he didn't kill him.
Everyone thought Katjaa took the cowards way out but she didn't. Yes Katjaa was scared to use guns, but she was like Tyreese who doesn't even know how to use one. All Katjaa knew how to do is be a doctor. Katjaa would have just left Duck in the woods if Nate wouldn't have showed up and killed her. Katjaa didn't believe in using guns, so why would she kill her self? If Kenny goes with her then he just finds her dead already. Kenny doesn't go into the woods until two minutes after the shot was fired.
This could play a role in season two. Kenny could come across Nate. Nate could mention he killed an old lady in the woods by the railroad months back, and Kenny could go off on him.
This is a twist too because we should know Katjaa isn't good with guns. She loved Duck too much to kill him. Nate our second antagonist in season two. Nate won't show up till episode 4.
Did I mention he's holding the same gun???