I dont have much to base this on, but IMO, its possible that TA came on really strong in the beginning of the relationship and JA believed their relationship was going to become permanent. He had so much to offer her that there wasnt anything she wouldnt do to catch him. Thinking that, she did her best to please him and become everything he could possibly want in a good Mormon wife. That being said, when he decided that she wasnt marriage material she still continued to want more and used all her wiles to catch him while pretending the sexual relationship was ok. Its my belief that she built up so many resentments against him, especially when he openly courted other women, that she finally made a last ditch effort and when that failed she had more in store for him than he expected. I believe this was a last ditch effort, a planned anger killing ready to be implemented if he didnt commit to her.