I too agree that the Cancun trip WAS in fact the catalyst. If you remember, Arias was SO arrogant and brash that she actually had the NERVE to be at Travis;s house "Cleaning" when he was out on a date with Mimi Hall. She then used the excuse of "Falling Asleep" to remain at his house when he came home from the date. While she acted like it was OK, her remaining there when he got home speaks volumes. She "Fell Asleep" so she could prove she could hang around even after he was dating other women. Then she leaves town, Travis is talking about Cancun, NEVER ONCE asking her to go, NEVER telling her who he was taking, because in trial she even is STILL speculating who he was taking, remember the "Babysitter", then she goes into wondering about Mimi Hall at the funeral memorial. That trip was more than she could bare, him NOT asking her to go, him going with another woman. She couldn't handle it, that trip is what pushed her to kill him, if she couldn't have him NO ONE else could either.