To open up an unobstructed lane of access for their drug shipments & mules.
Great summary.
Several replies have asked why they'd take and hold ranches along that lane.
The goal is to scare people off, not necessarily to replace them, yet.
They may want to stay in some US locations, not in a cluster, but they don't need to hold property.
With a couple of Arizona ranchers killed and some number of ranchers chased off their land by armed gangs fewer US citizens are likely to want to remain, fewer will be willing to confront the invaders, and the US government will remain oblivious so long as there are only 'scattered incidents'.