Most DV-related disappearances are not so simple. We have cases where the criminal tried more than once to kill someone and failed to follow through (or get accomplices to follow through). Patrick Frazee comes to mind.
None of their hikes in Utah were in the kind of forested land that is in Grand Tetons. Pretty easy to find a body at Arches - or Capitol Reef - or Canyonlands (Zion has some mazes with trees, but we have no word that either of them hiked in those areas in Zion).
Not sure why you think there needed to be planning, though. Things can build up and tensions can intensify, the longer the underlying issues aren't addressed.
Who knows how he felt about documenting the whole trip (which wasn't exactly happening as planned, as the posts seemed to have slowed and the new blog page is still under wraps).
It's the fact that she stopped texting family (including her cousins, with whom she was frequently in contact via SnapChat...) that's alarming. BF has offered no explanation in his "family statement."