Use of deadly force is authorized if at the discretion of the officer either his or other innocent lives are placed in danger.
Since we don't know if there were any immediate witnesses, and presuming this officer was seasoned and there NEVER had been such an incident at this location before, it is safe to say the officer used appropriate force.
Other questions:
Was the dirver a U.S. citizen?
Was the van stolen?
Many Mexican drug smugglers obtain US citizenship, it's easy and encouraged now to have both citizenships so US citizenhip is almost irrelevant. This is all part of the Mexicanization of the USA ----listen to their popular songs for example Los Tigres del Norte and other popular bands ---killing law enforcement agents is glorified---so much that the narco-corridos make gangsta rap seem naive. In McAllen last week a Mexican cop was shot down, it's very very common in Mexico for cops to kill citizens for the citizens to kill cops.