Here's a thing you can try some time in a game. Have one of the PC protagonists encounter an old friend. An old friend who is now working for the bad guys.
Maybe it's obvious right away, somewhat similar to Finn's situation here. The heroes are fighting or infiltrating the bad guys and one of the otherwise faceless mooks recognises one of the heroes as an old friend. They grew up together and went their separate ways to seek their fortunes, and this old friend just happened to find gainful employment working for an evil necromancer. Throwing an awkward personal encounter into an otherwise straightforward situation can really add a twisty bit of spice to the events.
Or... maybe it's not clear at first that the old friend is working for the bad guys. The heroes encounter the old friend in a street or market. The old friend is just out buying supplies or whatever, and they stop to say hello and have a catch-up chat. Maybe they agree to meet later to share a meal and reminisce about old times. You could even let this develop over a few days - until the heroes discover that the old friend is in fact working for the evil faction that they are here to destroy.
This sort of slow burn set-up can make the shock even more dramatic. Let the players even become fond of the old friend NPC before revealing what they're doing.
Commentary by Keybounce (who has not seen the movie)
[Keybounce's comments will appear here when received.]
Commentary by memnarch (who has not seen the movie)
A telescoping blaster cannon? That's pretty fancy for a plain-shoulder trooper. Oooo, or maybe it's a telescoping laser net-throwing cannon! I bet this got a neat little action clip in the movie. If it is a net though, I bet it won't do much flying against a laser sword. Assuming Finn can bat it out of the air without chopping something off. Annie is definitely playing Finn as less concerned about the plasma cylinder than Corey was with Luke. I give it low odds of cutting something off, but pretty good odds of still causing Finn an injury of some kind.
And that's a little harsh to say, fourth-panel-Finn. You could have at least said something like: "Then take them with you." Then maybe you could have escaped easier and started trying to take everybody down; the Resistance for sending Poe who killed Gil, and the First Order for tarnishing Gil's reputation. On the other hand, Finn probably isn't thinking completely straight now, so someone else might to need to talk him down from immediately trying to kill everyone here.
Man though, that lightsaber blade is really close to Finn's face.
Finn: The First Order gave me a spy job, then tried to reel me in and blame me when they messed up.
Finn: They’re corrupt. You gotta leave.
Zeppo: I... I can’t do that, man. My family—
Finn: Must be tough to still have loved ones.
Zeppo: I don’t want to hurt you, man. Come peacefully, talk with Lord Ren, and it’ll be fine.
Finn: I don’t think so.
Zeppo: Hang on. Is that... a laser sword?
Zeppo: Put it down, man. I don’t want you to hurt yourself either.