Neither is any more right or wrong than the other. It's also not necessarily the case that Luke killed the Gamorrean guards anyway. However, the fact that Luke uses a technique that has only ever been seen used by Darth Vader in any of the films (including the PT) is obviously meant to indicate that he's playing with fire, just as Obi-Wan told him on Dagobah - "this is a dangerous time for you, Luke, when you will be tempted by the Dark Side of the Force." It's a simple association, nothing more. There's nothing inherently wrong with a red lightsaber, either, but when Count Dooku first ignites his in AOTC, the fact that it's red is obviously meant to suggest that he's gone bad - because the only red lightsabers we'd seen previously were wielded by Darth Vader & Darth Maul. If the Sith Lords had used blue lightsabers, while the Jedi used red, you'd give Dooku a blue one. Still would have worked, the colour doesn't necessarily matter (although red's probably got more negative connotations than blue - danger, stop, blood, etc). They're just quick, efficient ways of making a greater point without wasting screen time on long exposition. Over-analysing them misses the point. In SW, a lot of the time you just have to take things at face value and accept them.