Why would the Stormtroopers want to make an "example" of the Lars? And how did they "give quarter" to the Rebels (I thought "give quarter" meant to spare their lives by taking them prisoner, but in this case it sounds like they're using the term "quarter" as in "quartering troops in your home")? Luke wasn't a part of the Rebellion at this point, nor was Owen, or Obi-Wan that we know of. The Stormtroopers went out of their way to make their slaughter of the Jawas look like the work of the Tusken Raiders.
Plus, with what they left behind, how would anyone know that this is what happens to Rebel "quarter-givers"? What evidence would they have that this had to be the work of the Empire?
And why would an aerial (orbital?) weapon be so weak?
This story is fishy...