I have completed all existing story content on multiple chars, but only now I finally got around to doing KOTFE and other stuff on my JK. Originally I planned to kill Arcann on him, but the more I advance through the story the less I am sure.

On one hand, this IS Star Wars. Good guys are always nice, baddies get redeemed. So I should just let Arcann complete his character's arc and development and let him become good, Plus I like and pity Senya, so... killing her is hard.

But on the other hand... i play my JK as a ruthless self-righteous warrior who shows no mercy to his enemies, and firmly believes that the only good Sith/Imperial is a dead Sith/Imperial (with Lana Beniko being the only exception because, well, it's Lana) Remember Lord Praven? My JK never bothered to try to turn him, he just killed him then and there. Same thing with the guy from Tython - dead. Arcann may not be a Sith, but he is an evil Force User, so same rule applies. The guy waged war against Republic and bombarded planets. He can't possibly deserve any mercy.

Then again, JK himself was once brainwashed by Emperor (who is Arcann's father and responsible for Arcann's being that way)into being evil, and he got better. That can make him sympathetic towards Arcann situation and drive him to help Arcann in turn.

So, yes. as you can see, I am pretty conflicted about this decision. Any thoughts? Ideas?

My advice is more general than this situation: don't get hung up on your plans. My first "real" character through KotFE was a F!Commando who romanced Aric Jorgan, then flirted her way into bed with Theron during SoR, and I was planning to have her continue the Theron thing in KotFE.

Except that she looked at Lana at the pool in the swamp in Chapter IV, and something joined up with something else, and I couldn't do it. So I clicked [Flirt] and the rest is, as they say, history.

Alternative formulation: let your characters grow. If that means they reduce their ruthlessness in some situations, so be it, just as in other situations they might kill someone they might have saved before. Or, indeed, switch from romancing men to romancing women. Whatever. Let them be who they are.

You can always replay the chapter.

You can, but doing so won't change the permanent record of what happened for the character.