As The Cousins were making their way into the US to kill Heisenberg (Breaking Bad, Season 3, Episode 1), they are at one point riding in the back of a truck (concealed within walls made from bales of hay).
A young man is being chatty, boasting about how he has made the trip 3 times, and has a job waiting. Nobody else jumps into the conversation so he directs his next comments to the cousins, noting how they have nice shoes. Then he notices the shoe tips have silver on them, looks close to see a skull, and suddenly falls silent (I guess recognizing them as being cartel members).
Moments later, the view pulls back to show the truck from the outside and shots ring out as the cousins supposedly kill all the other passengers. The driver, hearing the commotion, stops, goes to the back of the truck, and asks what is going on. They kick open the back, he starts to run, they shoot him as well.
Then they set the truck on fire, and leave.
Did they kill everyone because the young man recognized they were cartel? Did they always intend to kill everyone else? If the latter, why?