I just watched this last night, and two things struck me that were brought up in this thread. I'm not sure if Cato was running to get to the phone to call the police and collect the reward money, assuming that there was a reward, or if he was just running to the truck to get the hell out of there, but I was certain that there weren't actually any Spanish coins found in the field, and Kit may have killed him out of that realization, too.
The other thing is that Holly seemed a bit too detached from reality, even in the direct seconds after seeing her father being killed. I was trying to wrap my head around this, plus the simpleton tone and therefore, as someone stated above, the unreliability of the narration, and I thought that maybe it's not Holly's narration, but Kit's imagining of Holly's version of the story. If you've seen Charles Starkweather, he was a weird looking guy, but that character imagining his own story would picture himself, and his girl, as looking like James Dean. He's kind of a dummy though, so he would have a certain limit to how bright Holly would've been. In the real version of events, Holly would've probably been visibly upset about her dad being killed, but in Kit's memory it was probably more like "oh, right, I guess she was sad for a second."
And, obviously, great cinematography and music, except for the multivoice music that came on a couple times, though I can't exactly remember the scenes.