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Boba Fett was on Tatooine at the time
According to 1997's Star Wars Special Edition, Fett was on Luke Skywalker's home planet when his aunt and uncle were murdered. This means he had the opportunity, the ability and (as we'll find out in a minute) the intent.
"These blast points, too accurate for Sandpeople”
According to Obi-Wan Kenobi, the Empire murdered the Lars but made it look like the Tusken Raiders did the deed. But why would the Empire need to cover their tracks? Kenobi deduced that Stormtroopers were on the scene, but we known that Stormtroopers have terrible aim. The shots fired against Owen and Beru were precise, making it more likely that Boba Fett, who's had more training (and success) with a blaster, pulled the trigger.
The Stormtroopers were behind in the chase
The Stormtroopers were still checking the cities, setting up check points and going through bars when Owen and Beru died. If they knew the droids holding top secret information were in the Lars' home, they would have set up camp around the house and ensured they did not get away. Instead, they were spread out through the city and likely the whole planet searching while Boba Fett went in for the kill.
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Tags Boba Fett Owen Lars Beru Lars Luke Skywalker Han Solo Obi-Wan Kenobi R2-D2 C-3PO Tusken Raiders The Empire Tatooine The Death Star