Two things to think about: Firstly, it might be worth asking if the players need to have a conversation. One player character attacking another is a pretty serious state of affairs and you need to watch out for an underlying conflict, or a situation where one or more players have an OOC problem with the way your Captain player is behaving IC. Sometimes a player goes too hard into “it’s what my character would do” at the expense of other people’s fun.
Secondly, if there isn’t an OOC problem between anyone at the table I don’t see any reason you couldn’t resolve this all ingame. In one of my campaigns, the chief medical officer got brigged for disobeying direct orders from the XO (their characters hate each others guts). The CMO is pretty far on the renegade side for a Starfleet officer, so she immediately made friends with a criminal also in the brig who the security chief had failed to interrogate, got her to make a deal and so got released without charge. This all happened in character without me having to intervene! Even though one character firing on another is more serious, it’s still largely up to the characters what happens next (and you just have to prepare for all the various possibilities )
- Will have probably multiple negative reputation influences for this
- The captain will need to decide how he wants to handle the situation - it’s an officer under his command, so it’s his call rather than Starfleet Command (i.e. you)
- The Captain’s player needs to understand the stakes: charging and court-martialling the engineer could permanently cost the player his character (or if the court finds the Captain’s actions wanting, could backfire)
- Brigging, reduction in rank, reprimand all options of lesser severity
- One of the hallmarks of roleplaying games is that player actions can have serious, lasting consequences. Rash actions should have results. Nevertheless, literally taking away someone’s character is a big, big step, so as GM you may want to find a way to avoid that rather than necessarily prioritising a ‘realistic’ outcome
- Negative reputation influence(s) can represent Starfleet’s quiet disapproval and increased scrutiny.
- I’d say this is a matter for a stern communique from an Admiral rather than a board of enquiry or anything. Not all consequences have to be instantaneous - a crew might be quietly shuffled to duties of lower importance and have to win back Starfleet’s confidence. You can give them those opportunities - stumbling across a serious situation or threat on a boring and “safe” survey or patrol mission etc.
- With that said, Starfleet has no shortage of Captains (and especially Admirals!) who favour a more… ‘ends justify the means’ approach. Your Captain may not be acting in the best traditions of Starfleet, but it may be that Starfleet Command sees his solution as a necessary (if unfortunate) compromise of the ideals of the Federation.
There’s one other possibility to prepare for of course. If enough of your senior officers decide to side with the engineers, you have a proper mutiny on your hands and they can choose to remove him from command. There are a load of interesting storytelling opportunities there but you’ll need to decide how you want to handle that outcome.