I've given this quite a bit of thought. Here's what I think happened. She went there with the intent of "reminding" him of their sexual compatability. She had already done everything she could to convince him to continue having a serious relationship with her by being a chameleon. That is by changing her personality to become exactly what she thought he wanted in a full time girlfriend, future wife, etc.
And she just could not fathom how he could reject her (not sexually, but by denying a serious, committed relationship) when she became everything she thought he wanted.
So in a way, this was a last-chance visit. So when TA restated that he was going away without her, JA came prepared. She tricked him into the shower photo taking - I believe he was actually posing in the pictures. When she got him in the most vulnerable position, sitting on the shower floor, she ran forward and stabbed him. She planned to do the stabbing in the shower so that she could wash off any traces of her DNA.
But instead of becoming incapacitated, he ran over to the sink, spitting blood. I believe this caught JA by surprise. I think she then started stabbing him in the back, while he was bent over the sink.
But he got (crawling perhaps) away from her, and he may have been screaming. And in fear of anyone hearing him screaming, yelling, whatever, (JA may not have known whether a roommate had come home at some point) she panicked and ran after him, slitting his throat to stop him from screaming. Weakened now, but maybe still barely alive, she drags him down the hall back into the shower. Since the bullet casing was found on the bathroom floor, but not in the actual shower stall, she may have shot him there for good measure. It's hard to say when that gunshot happened. Maybe in between the stabbing sequences.
Once she got him back into the shower, I think we all know she rinsed him off, tried to clean up the floor etc etc.
Those are my thoughts based on what I've heard so far. I could be completely wrong of course!
What is hard for me to grasp though, is how anyone, JA, or anyone, could systematically and thoroughly attempt to clean up the scene - if I saw that much blood myself, I would faint on the floor. I guess it takes some kind of sub-human to overlook that.