You seem to really not want to say anything about how multiple soldiers were seen involved. There could be more than one person. How big of a conspiracy it is, I don't know. It might just be a unit of Green Berets who felt like killing people. But it doesn't seem like a lone crazy. Either that or Karzai is lying.
The current version tells of one gunman. Let's stick to that. Testimony of a terrified 15 years old boy doesn't say much, even if he did really say that. He could be used to the fact soldiers always travel in groups, so when he saw one, he thought there are more behind him. Considering that he survived, he most likely hid well shortly after seeing the soldier. Remember that the whole incident must have scared the living daylights out of him, so I'd hardly consider him a credible witness.
Karzai may be lying, but I don't think it's the case. The situation is bad enough, I don't think he'd see need to worsen it (and risk being exposed as a liar). He's most likely honestly believing that boy, but as I said, his testimony might not be very credible.
Of course, there's a chance a bunch of soldiers felt like killing some Afghanis for some wicked reason. Since I don't think they could all have PTSD manifest like this (though depending on the size of the group, it's not impossible), that'd make them a particularly nasty kind of murderers. And they'd have to be threated as such. Though I sincerely hope it's not the case.
