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[Star Wars] What exactly happened to Aunt Beru and Uncle Owen?

So I watched episode IV for the first time in forever, and while it is an amazing movie (as always), one thing bothered me: the deaths of Aunt Beru and Uncle Owen.

Now, we know *who* killed them: Imperial Stormtroopers hunting for droids that have the plans for the Death Star (C-3PO and R2D2).

However, there is still the *why* and *how* they were killed that throws me for a loop. 

We see a Jawa Sandcrawler taken out. Obi Wan comments that "Only Stormtroopers could be so precise" and Luke comments that this meant that the droids could be traced back to...Oh no! We are then treated to Luke getting back to his home and seeing two charred skeletons. 

Here's my problem: There is no indication that the Stormtroopers had any incendiary weapons on them. The Jawas killed by them were all shot. So why/how were Beru and Owen burned to death? 

Why were they killed in the first place? The Empire does rule by fear, for sure (I mean like 10 minutes later we see Aldaraan blown to smithereens). But why unnecessarily execute these two farmers? Was it due to the memory wipe? I feel like the Stormtroopers could have been all "Hey we think some droids were sold to you, any idea what happened to them they have something of ours" and then the whole misunderstanding is cleared up. Instead the Stormtroopers were like, "WHERE ARE THE DROIDS. NOT HERE? KILL THEM BOTH. NO DON'T SHOOT THEM SET THEM ON FIRE FOR SOME REASON."

Bothers the fuck out of me. What actually went down?
gpt
They were probably shot, then set on fire.  Kill the people, then destroy the farm.  Burning down the farm included burning the bodies.  
As for why, it's just taking an aggressive stance on the search.  Remember they're dealing with Rebellion leadership and stolen plans for their superweapon.  Everyone involved with these droids is an enemy, and is treated as such.  It's an unsubtle approach, but the Empire is not looking for subtlety here.