The Mandalorian Season 2

[Aeson]

The problem with your argument for why Luke should be morally justified in killing the Emperor is that you are constructing a rationalization based on potentialities ("if the Emperor does not die by Luke's or Vader's hand in the throne room, he might survive the battle of Endor and go on to inflict untold harm upon the galaxy") and factors of which Luke is, to the audience's knowledge, unaware (the Emperor is a significant and immediate physical threat, not just a moral one). The only warnings Luke has been given regarding the Emperor's power are vague (things like "do not underestimate the Emperor's power" and "you do not know the power of the Dark Side of the Force"), and on top of that the Emperor presents himself as old, physically unimposing, unarmed, and reliant upon Vader for protection against Luke when virtually all previous encounters between Force adepts - especially in Luke's experience - have been rather physical in nature and involve weapons; additionally, Luke knows of Palpatine's success in corrupting Anakin and interest in corrupting him, and has also been warned several times that the Dark Side is an easy and seductive path, but not one which grants greater power. Thus, given what Luke sees of the Emperor, what Luke sees of the Emperor's behaviour, and what we know Luke knows of the nature of the Dark Side and the Emperor's past history of corrupting those around him, it would be reasonable for Luke to think that the threat of which he has been warned is largely, or perhaps even entirely, moral in character. Furthermore, we know that, at least on his arrival in the throne room, Luke expects the Rebel attack to succeed and it is strongly implied that he thinks that the Emperor will die as a direct consequence of that, rather undercutting the argument that in killing the Emperor Luke would not be acting immorally because he would be preventing potential future harm.

Also, arguing the morality of an actor based on the potential future outcomes and consequences of their actions is very questionable, and while it is in human nature to cut someone slack when their actions produce positive outcomes and judge more harshly when their actions produce negative outcomes that really doesn't have a lot of bearing on their morality.

Um...yeah, the point is that the Force only considers the 'morality of the motivations' not the morality of the actions themselves.

Is that actually true in Return of the Jedi, though? I do not believe it is; the Emperor does not appear to be a physical threat to Luke up until the point where he starts to electrocute Luke, nor is there any clear evidence that Luke should have seen him as such given what the audience knows Luke to know from watching the Original Trilogy. That being the case, Luke killing the Emperor is on very dubious moral grounds - the Emperor seems to be merely morally repugnant and corrosive, not a clear and present danger, and killing things that are morally repugnant but not immediately dangerous is not a moral act, at least in the context of modern morality.