Hi! OH MAN THIS IS KIND OF A LONG SUBJECT BUT ONE I LOVE.
For the immediate question you’re asking, I go pretty much with what George Lucas says in the making of ROTJ book:
Ben lied, but he didn’t really lie. That might seem like splitting hairs or getting into semantics, but I think getting into that discussion that misses the point of what I feel George is saying here, that despite technically lying, Obi-Wan was telling Luke an emotional truth, he was telling him what he was ready to hear. That, in an earlier version of the script, George specifically wrote that Luke wasn’t ready to be told.
I mean, it’s a pretty big thing! Not only is it a difficult story to wrap one’s head around with only half of the context, but it’s a huge burden to shoulder, the weight of having this connection to someone like Vader and trying to balance that with how he’s going to try to use Luke, how the Emperor will try to use it against Luke as well.
The lack of being ready for that is still woven throughout ESB and ROTJ–when Luke goes to Bespin, he finds out the truth and is so devastated by it, he falls off the balcony rather than stay with Vader, in a move that was basically suicidal. Mark Hamill has talked more than once about how that’s what was going on with Luke:
That’s how little Luke was prepared to hear that. To the point that, going to Bespin went so badly for him, that Lando and Leia and Chewbacca missed their chance to go after Boba Fett (who had Han in carbonite on his ship) because they had to go back for Luke to rescue him.
So what else happens when Luke finds out, beyond that moment of potential suicide? Well, we’re seeing it play out currently in the Star Wars ongoing by Soule, where in the immediate aftermath of Bespin/the parentage reveal, Luke has been shown to be dejected and his connection to the Force is so wonky that even Lando calls him out on it, saying that Luke and the Force aren’t on speaking terms right now.
This is a page that’s deliberately evoking our memories of that iconic moment of Vader finding out the truth about the pilot who blew up the Death Star: Luke Skywalker.
Vader’s rage is so great, as he realizes what this means, that he cracks the glass of the ship around him. Now, Luke is finding out and he does the same thing!
His connection to the Force–and connections to the Force are based on emotion and mental stability, it’s all about how you approach it, whether you’re selfish or selfless, whether you’re calm and compassionate or angry and afraid, that’s literally How The Force Works–is in a really bad place showing that he was not ready for this and, even if we know he’ll find peace with it eventually, it wasn’t the right time or the right way to find out.
This doesn’t mean that Luke isn’t valid for feeling upset about not being told the truth, that’s what the entire conversation with Obi-Wan in ROTJ is about, but Luke also needs to recognize that truths can be subjective, that there are different kinds of truths–technical truths and emotional truths, that sometimes you’re not ready to hear the full truth yet, because it would be too much.
Even before all of this begins, in From a Certain Point of View, Obi-Wan has a conversation with Qui-Gon’s ghost about this, how he says Luke will have to know the truth someday, showing that he never intended to hide it forever.
Qui-Gon responds with that Luke may have immediately been lost, that the whole story would have just confused the hell out of him and led him into a spiral.
Even in Return of the Jedi, when Luke goes to talk to Yoda about the truth being kept from him, they have the following conversation:
LUKE: Master Yoda… is Darth Vader my father?
YODA: Rest I need. Yes. Rest.
LUKE: Yoda, I must know.
YODA: Your father he is. Told you, did he?
YODA: Unexpected this is. And unfortunate.
LUKE: Unfortunate that I know the truth?
YODA: No. Unfortunate that you rushed to face him… that incomplete was your training… that not ready for the burden were you.
LUKE: I’m sorry.
It’s not unfortunate that he knows, but that he wasn’t ready for it. They always planned–or at least Obi-Wan did, but it’s a reasonable assumption that Yoda agreed as well, it’s not like they didn’t think that Vader wouldn’t tell Luke the first time they met, either, so of course they’d tell Luke eventually, when he was ready for it.
As well as, no, their intention to use Luke to kill Vader was not their goal, George Lucas talked about that in the making of ROTJ book, too, talking about the set up for the scenes when they were storyboarding:
“The mission isn’t for Luke to go out and kill his father and get rid of him. The issue is, if he confronts his father again, he may, in defending himself, have to kill him, because his father will try to kill him. This is the state of affairs that Yoda should refer to.”
They were right that Luke wasn’t ready to know the truth, that he was at a dangerous point in his training where he was especially vulnerable to the dark side, and that’s why they kept the truth from him. But that they always intended to tell him when he was prepared to hear it and understand the bigger truths, rather than just hearing, “Oh, yeah, your dad is alive, but–” and then not listening to another word because Luke wasn’t yet that solidified within himself.