GREAT idea!!
Some time ago, I started a thread about Court Martials as focus of an episode/adventure. You might find some interesting responses, there. The thread originated in a thread about excessive force being used, which also kinda relates to your topic…
Regarding the case at hands: Judging from what I’ve read in your post, we’re talking outright murder, if not the war crime (!) of killing a prisoner of war without due process. I see no way this will not end in decomission and imprisonment (for a long time).
In case you do not want to kick out a player out of the campaign (which, honestly, would definetly be fitting, in-game), you should, in my humble opinion, think of some deus ex machina:
- Maybe the Gul isn’t dead and vaporised but transported away, like Picard was in TNG 7x04/05 “Gambit”? Maybe, Starfleet Intelligence has him for further inquiries? Or maybe even Section 31? Maybe your chief-of-security thought they were working for Starfleet Intelligence, but ultimately were only a pawn by Section 31?
- Maybe the android suffered severe malfunctions so there’s the possibility of some kind of insanity defence or reduced capacity or something like that?
- Maybe the android was taken over by a plot of someone else who wanted to see the Gul dead? Examples could include the Obsidian Order, killing off a liability while blaming a reputed Starfleet Officer.
My advise would be: Draw as much Drama out of the situation, as possible. Have the character confined to their quarters (or the brigg) for a mission or two and let their player take supporting characters. Maybe even their own legal defense, working out a strategy with the rest of the crew to get back their friend. During these missions, let the players find out the motives and what really happened. This could involve a detective story or engineering examinations, or both. It could mean they would need to capture a foreign (or, worse, internal) spy and uncover a plot against the Chief-of-Security, the Ship, Starfleet, or even the Federation.
After the crew gathered evidence for a mission, or two, go to the Starbase and set up your court martial – which is, again, a great idea. I did this with my D&D group some time ago; you can read my experience in the first thread I’ve linked in this post.
In ANY event:
- Make sure to include every player in your group, including, of course, your chief-of-security in the proceedings. Supporting Characters are a great way to achieve this.
- And, please TALK to the player of your chief-of-security how they would handle this situation. Maybe they wanted to change character, anyway? Maybe they’ve got some great idea how their character can go through the proceedings, unharmed? Make sure that everything that happens is okay with the player. Neither do you need to tell them everything, nor do you need to tell the rest of your group something.
But court-martial episodes, with demotions or even a character that is now no longer playable are difficult situations. Make sure that all the drama stays in-game!