'American Sniper' : Reality about the movie's end? [closed]

Kyle's accused killer, Eddie Routh, has never stated why he killed Kyle and another veteran, Chad Littlefield, that day. All that's known is that Kyle and Littlefield took Routh, who suffers from PTSD, to the gun range that day, and that a hunting guide later found Kyle and Littlefield dead on another part of the property. Kyle's truck was missing, and Routh was nowhere to be found.

UPDATE

I happened to stumble over this old answer and did a quick search; Routh later admitted that he shot the two men on the way to the range because "they wouldn't talk to me. I was just riding in the back seat of the truck and nobody would talk to me. They were just taking me to the range so I shot them. I feel bad about it, but they wouldn't talk to me. I am sure they have forgiven me."