I think it's inexplicable for EU Luke, was like Super-Luke.
I don't think it's inexplicable for a Luke who came from the OT but didn't do all the EU stuff.
This is where I think the disconnect is. People see this saintly, super-experienced EU Luke, who has had countless adventures as the good guy, but that's not the scenario here I think.
It's not "morally gray".
Bringing that up suggests you aren't understanding the scene. The film's position is that Luke would have been entirely wrong to murder Kylo Ren, even if it's what Prequel Era Jedi would have done (and it is what they would have done, by and large).
No. That's literally, textually wrong. I know neither of us have seen the movie for a while, so here is the scene:
It is absolutely described as a momentary lapse in judgement!!!! ALMOST LITERALLY! Luke says "and with the briefest moment of pure instinct".
So let's be clear, the sequence of events is:
1) Luke realizes Ben Solo is falling to the Dark Side.
2) Having failed to stop this, Luke goes to Ben Solo's room to do some kind of Jedi mind-scan/soul-scan to see how far he has fallen. He explictly has not come to kill him. His lightsaber is not in his hand, he's there to find out.
3) Luke BELIEVES that he sees that Ben Solo has completely and totally fallen to the Dark Side, and serves Snoke. This is surprising to Luke and he wasn't expecting it - that shows you are proven wrong when you claim Luke was planning a murder. Don't take it too hard, you haven't seen the scene for a long time, neither had I, but that's a fact! Luke would not have been surprised if he was planning to kill him, he merely would have been saddened.
Now believes is important here, because if you watch the scene, it seems like Luke might think he was tricked - presumably by Snoke or the Dark Side itself, perhaps exaggerating how far Ben Solo had fallen.
4) "and with the briefest moment of pure instinct" - i.e. a momentary lapse of judgement - Luke draws his lightsaber and considers striking down Ben Solo. Only considers, and only in that moment. This was textually not a planned murder.
5) Ben Solo wakes up and is like "WHAT THE F*** MAN?!?!" - and Luke realizes he's just a scared child. This again is indicative of Luke being in some what tricked by the Dark Side of the Force, because he goes from seeing this terrible threat to "just a kid"
(Now we might want to recall that this bloodline has something of a habit of killing kids... just saying... them younglings!)