LOL they're freaking natzi rednecks. Killing that kid in the desert didnt' make sense. Arrogance played a huge part in that entire scene. It played a huge part in the entire purpose of the Natzi's. Todd told them about the meeting with Lydia, how he had spent all his money trying to get back, how he was trying to basically get back into the business. He gave them information in hopes that they woudl drop their guard, and they did. How is that a plot hole?
The fact of the matter, it is totally plausible to happen in that situation. They underestimated Walt. Plain and simply. Just because YOU find that as a plot hole doesn't mean it truly is. Hell, he didn't look UNDER the car to find a bomb. They didn't look under the hood either. They didn't pat his ankles down either.
I mean shit dude.....
Don't be a tool.