There is no scene put into Star Wars that firmly establishes Owen and Beru's killer(s). The murders can actually be reasonably deduced as the work of bounty hunter Boba Fett.
Darth Vader established in the death of Captain Antilles that interrogations ending in death should be done by Vader alone.
The stormtroopers follow orders to stun Princess Leia. Why would they be ordered to kill Owen and Beru?
Searching for the droids is a high priority. Vader has been known not only to use Imperial Stormtroopers but also bounty hunter to accomplish goals.
In the search for the droids, someone was clearly on a killing rampage.
How were the stormtroopers gathering information? They were setting up check points. They were following orders because they were incapable of doing anything else.
Who was Boba Fett? Boba Fett was a clone of Jango Fett. Boba was simply an unaltered clone. Boba was designed to not be docile. Boba was designed to not follow orders when orders did not suit his purpose.
Who are Stormtroopers? They are altered clones of Jango Fett.
Even if he had too, Boba Fett could have easily worn Stormtrooper armor and masqueraded as one easier than other people. After all, unlike some people, Boba Fett was exactly the right height and build.
Even if someone clearly saw a stormtrooper kill Owen and Beru (as well as the Jawas), the act could have been the easiest deception in the world for Boba Fett to pull off. After all, Fett was clearly at the scene of the crime.
The next time we see Fett in the series, he is warned by Darth Vader specifically not to disintegrate.
Vader may have been the baddest man in the galaxy, but Vader always took prisoners when straight up murder was a possibilty.
Funny, she did not seem Druish.
Other than Capatin Antilles, Vader generally only killed in response to incompetence.
Darth Vader had a memory or every perceived wrong going back decades. When neccessary, Vader was more than willing to confront and deal with the past... personally.
Vader fully knew who his adopted family was.
He knew Owen and Beru. They held potential information that Vader would never want to be told to a Stormtrooper. The Stormtroopers would turn on a superior in a minute if they were given a higher order. They would not disobey an order. There is very little possibility that (even if Vader wanted Owen and Beru dead), he would have let a Stormtrooper handle it. This is especially true after the capture of Leia. Vader would have wanted a personal conversation with Owen Lars.