Hmm, the Knights of Ren are a tough nut to crack.
On the one hand, Luke’s dialogue in TLJ about Ben taking a handful of Luke’s students with him made me think the other Knights were Luke’s other students. On the other hand, I think it would be weird to depict them as such when TROS does nothing to suggest that. They apparently were Jedi students, but none of them wield lightsabers or ever display any Force abilities. Plus, you’re left with the weird conclusion that Ben was responsible for their turn, but did nothing to help turn them back and instead killed them all when it wasn’t convenient for him anymore. If Ben even had one line on Exegol, trying to turn them back, that probably would’ve sufficed.
If the Knights were just Sith or dark side cultists who follow Kylo Ren because he is the grandson of Darth Vader, it would probably be a cleaner explanation that doesn’t leave you scratching your head as much. They dress the way they do because they worship the Sith, and of course they would choose Palpatine over him once he returned. Sure, maybe it isn’t as interesting as the Knights being some of Luke’s other fallen students, but it would be a stretch to make that clear, with what little material we have to work with.
BUT, since I hate myself, I’ll try to lay out some of my own thoughts for how one could try and make the Knights of Ren be fallen Jedi as clear as possible.
First of all, here are some suggestions for dialogue. I thought the Knights would be a good way to explicitly establish Kylo’s feelings for Rey early on, as well as be a way to show the Knights feel like Kylo has betrayed them for her. Some extra dialogue could hint at their force sensitivity and their relationship to Luke.
Mask reforging -
Knight: Your feelings for the girl are a weakness. Bury them deep. Then we’ll bury her. (or, referring to the mask) This will remind you of who you really are.
On Kijimi -
Knight: They’re close… I can sense them.
or Knight: The girl is close… I can sense her.
On Exegol -
Knight: So, you were seduced by the girl.
I think it would be great if Ben could say something here, like, “Don’t.” “Stop!” “Join me.” “No.” At least give Ben some semblance of an attempt to talk to them.
Knight: Sidious has offered us more power than you ever could. (or) Say hello to Luke for us.
I may have mentioned this on the TROS main thread, but I think if you were to make them Luke’s students, it might also be worth trying to make some of them carry lightsaber-type weapons. If they were former Jedi, it makes sense for some of them to have Jedi-esque weapons.