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A good offence is the best form of defence, after all.
If you want to defend a town against a horde of goblins, a good plan might be to attack the goblins first. Assuming you have advance warning, of course.
But does that make you the aggressor? You can try this as a moral quandary for your players. They hear rumours of an imminent planned goblin attack. They could get the jump and launch an assault, but generally speaking the goblins just keep to themselves and don't really hassle the kingdom, and these rumours... how reliable are they? Where did they come from? Do they attack the goblins to avert possible danger, or do they dig deeper?
Commentary by memnarch (who has not seen the movie)
Blue?? Blue?!??? Why'd he have a blue lightsaber??
Ok, initial reaction aside, I suppose blue makes sense since Anakin also had a blue lightsaber before he was tempted to the Dark Side. But still, this was really weird to suddenly see.
And I suppose since there's an actual attack by Luke here that this is probably more than just a vision of some kind. Which makes me wonder what Luke might have realized or thought to attack Kylo like this. Unless it is a vision, and this is just what Kylo is remembering the scene as to justify things to himself. We already had the Force Pushing shown, which means either the lightsabers go missing which is rather unlikely, or something else is going on.
As for "Why would it do that?" I pick Narrative Reasons. The GM loves planning lots of things out even if the minutiae of those plans tend to be lost when the PCs get involved. Sanity saving would be just a bonus.
Transcript
Kylo Ren: Luke would have killed me while I slept.
Rey: Liar!
Kylo Ren: Believe what you want. He swung at me with a laser sword.
{flashback scene showing Luke with a green laser sword; Kylo and Rey speak over the top}
[SFX]: whoom!
Rey: You knocked him out, then burnt down his Academy and slaughtered the students!
Kylo Ren: In my defence... that was in my defence.
[SFX]: kzzrzkzk!
Rey: How do you figure that?
[SFX]: whmmm... {young Kylo holds Luke off with his own blue sword}
[SFX]: kzzrzkzk!
[SFX]: force shove! {young Kylo shoves Luke back, destroying the hut they are in}
Kylo Ren: Do you have any idea how hard it would be to rise to power with a school full of Jedi hunting you down?
Rey: Mmm... Fair.
Kylo Ren: Sounds like you have the makings of a Dark Side crusader.
Rey: Nah. The Dark Side rejected me.
Kylo Ren: That’s... Why would it do that?
GM: Narrative reasons. Game balance. My sanity. Take your pick.