Here’s why I just don’t have a problem with Luke momentarily thinking that killing Ben Solo was the right course of action – especially in light of his (seemingly) contrary attitude with respect to Vader in ROTJ. Let’s look at Luke’s history:
Luke grew up believing he had no father; when he found out the truth about his father, that undoubtedly awakened some hope in him that, if Vader was in fact his father, Vader couldn’t be all bad and perhaps could be turned to the light- in effect, allowing Luke to regain the father he never had.
So what’s different between the Vader and Ben situations that might trigger different responses in Luke? Several things:
1. Let’s recall that Luke’s emotion toward his father was apparently reciprocated – meaning that, once he saw Luke (and, one assumes, also saw shades of Padme), Anakin himself began an internal struggle. This is most evident when Vader lets his emotional guard down in ROTJ and says “it’s too late for me, my son.” So Vader was giving outward evidence of this struggle – telling Luke that Luke’s hope was not completely unfounded.
We don’t have any evidence from TFA or TLJ (really, one way or the other) that Kylo was evidencing such a struggle at all. All we know is that he has unprecedented power (Luke to Rey: “I’ve seen raw strength like this only once before”), leaned toward the Dark Side, and based on the timeline (Han and Leia specifically reference Snoke as being instrumental in Ben’s fall), it sounds like Luke, Han and Leia were aware that Snoke was active in Ben’s life at this time. So, by the time of the contemplated killing of Ben, we have more than just a “student teetering on the brink” and likely more “an immensely powerful student being affirmatively groomed by a Dark Side user”.
2. Let’s face it – post ROTJ, Luke has suffered through the horrors that two Dark Side force users can wreak on the galaxy and – whether right-headed or not – is likely to take a very knee-jerk approach to making darn sure it doesn’t happen again, especially not where he might play some fault in it happening. (In fact, I'd bet Luke has feelings akin to PSTD when ti comes to the Galactic Civil War). Add to that, in his perception, it could be worse this time around, given Ben’s youth and that his strength in the force eclipsed Anakin’s. So it seems entirely conceivable - even consistent – that Luke’s first impulse would be to take the most drastic action necessary to prevent the galaxy from plunging into an even worse darkness than before. So he starts down a rash course – but, true to the Luke we know, he hesitates and thinks better of it (perhaps recalling the redemption of Vader); unfortunately, through poor circumstance, his initial impulse cannot be “taken back” and ends up making things worse.
So, yeah – I get that if Luke had gone through with his plan to assassinate Ben without remorse, that would seem out of character for Luke. But that’s not what happened – instead, he started to make a bad choice – based on factors and circumstances that are consistent with what came before – but his better nature prevailed and stayed his hand. In other words, Luke started to make an understandable but bad decision and stopped himself – how is that “out of character” for any of us, much less Luke?
M
Yeah, and I would go on to add that the entire Skywalker family is very problematic in regards to the Force. Whenever a Skywalker comes to prominence in the Force the Dark Side rises to meet them. Just like Snoke said. I think Rey is there to fix that cycle.