After the testimony yesterday about Travis' computer being accessed shortly before 5 pm (presumably while Travis was getting ready to take a shower, since he would be dead a little over a half hour later), I formed a new opinion in my head about why she killed him with a knife instead of the gun she brought with.
I believe her original intention was to kill him with the gun - until she got on his computer. According to her testimony, TA told her he was taking "a babysitter" on the Cancun trip instead of her (which I think may have been true, if he didn't want to "set her off" by telling her who he was really taking).
I think she believed that by seducing him, that he would change his mind and take her instead (hence, her purchase for 2 cans of sunblock from Walmart), since it wasn't even someone he was close to going with him - or so she thought.
IMHO, while TA was upstairs getting ready for his shower, she got on his computer, and found that the tickets were actually for TA and Mimi - not some babysitter. She was furious that he lied to her, and that he wasn't going to change his mind about taking her with him because Mimi was TA's future wife (or so she claims he told her).
She quickly formulated the idea to do a photo shoot with TA posing as in the Psycho shower scenes, which would give her a reason to have a knife with her. She probably told him she would leave if she let him take these last photos of him. I think he realized at the last minute that she was going to kill him. I think the photo of him on the floor of the shower was actually her dropping the camera to stab TA in the chest, and that she was kicking the camera out of her way - which accounted for the ceiling shot, plus the shots of her dragging his body. I think she intended for him to hurt as much as she FELT hurt her emotionally.
I also think, based on the outline that the tech guy drew around the reflection in TA's eye shows that JA was holding the camera waist high in one hand(if he was sitting in the shower at that time, the flash could have momentarily blinded him from that height), and her other arm was reaching behind her for the knife.
IMHO, as I posted a few pages ago, I think the gunshot was for good measure to make sure he was really dead, since his eyes were still open.