I agree with you, but I don't think that photo is when she pulled out the gun or the knife. I think that's when she turned on the camera's flash, and he realized she was there and it startled him (maybe annoyed him to a certain degree too). The shower door is still closed in that photo. You can see the water droplets on the door and running down the door. I think that the very next photo, of his lower body sitting in the shower, is when he probably knew what was coming - the door is open in that picture. But his face is not shown in that picture, and that's maybe a good thing.
Many keep asking why he was sitting in the shower. After studying the pictures she took of him in the shower, and that final shot that shows how unhappy he looks, I think he sat down because he was just tired of it all. He wanted her gone, and she just wouldn't go away. The previous shots are not poses IMO. He looks like he's just trying to wash himself clean, physically and maybe metaphorically too.
I have never in my life seen "nekked" pics where the subjects look so apathetic. Usually there's the gleam in the eye and a look on their face that says "I'm goin' for it" (anyone remember the Tommy Lee/Pamela Anderson fiasco - now there was a dude that was "going for it"). I don't see anything like that in these pictures. It all looks forced and not natural at all.
I think TA was a victim of sexual abuse, and I might be roasted for saying that but it's how I feel. Anyone remember being disgusted at the Debra Lafave case? That kid was mesmerized at what he'd never experienced before. And what about the Mary Kay Letourneau case? She seduced a 13 year old, had two of his children while in prison, and married him when she was released.
I know Travis was not a child, but if he was a virgin as others say he was it's not a huge stretch to imagine the power her sexual advances might have had over him. And from the glimpses we've seen of the kind of person she is, it's not a stretch at all to see how she would have taken advantage of that.