start at the 1:55 mark, you'll see
Kyle shot Rosenbaum in the back. That's what that video shows dude...
You're jumping to when Huber attacked him, but by that point, even in the video you have here, it is clear that Rosenbaum was shot in the back because he FALLS FORWARD ON THE GROUND.
Also, Rosenbaum was unarmed, so Kyle had no fear for his life in that moment.
Notice all the witnesses around there too!