All the rhetoric that portrays these migrants as an invasion stokes the fear in enough crazy people, that this was bound to happen sooner or later.
All the rhetoric makes some Latinos (who were themselves once immigrants, or the children of immigrants) look down upon other Latinos.
There is also the criminal element. Gangs have become more active. This guy could be from a gang that works as a smuggler that took out his rage on these people. When a lot of money is to be made by shady people, things tend to go down in horrific ways.