He did that AFTER he believed Vader could be redeemed. And the point is that even despite believing in the good of those who have done evil things he can still be swayed away from that based on outside factors. Luke snaps when Vader taunts him with Leia and he contemplates murdering a boy because of his raw power and his belief that the corruption is too far gone.
You're equating raging at someone who taunted him in the middle of a duel to deciding (presumably after thinking about it for quite a while) to murder a defenseless boy in his sleep. Those are wildly different circumstances. One does not mean that the other is also in character.
I wasn't arguing it was his fault. But regardless of whose fault it is if you feel a young boy will become something that will cause immeasurable pain and suffering you would not even consider putting a stop to it? That is a trope that has been used a few times in recent memory, namely Looper and Deadpool 2.
Whether it makes sense or has been done in other franchises is beside the point. What matters is what Luke, a noble Jedi, would do. George Lucas based the Jedi on the samurai, not just in their swordplay, but in their code of conduct. Like samurai, Jedi don't kill defenseless or unsuspecting people because it's not honorable. After he chops off Vader's lightsaber-holding hand in RotJ, Luke doesn't finish him off. He switches off his lightsaber and turns his back to him.
For 40 years, Luke was the quintessential Jedi. Even if every other Jedi fell to the dark side or broke the Jedi code, Luke could be expected to stick to the Jedi principles and always do the right thing. While he does ultimately do the right thing in TLJ and doesn't kill Kylo in his sleep, it's out of character for him to even consider it, especially since he's the protector of the ancient Jedi texts that presumably contain that rule and all others that he lives by.
I think that what would've been more in character would've been for Luke to confront Kylo and kill him in a duel, if absolutely necessary, or, an even more noble of a solution: pursue and defeat Snoke, the source of the corruption. Murdering Kylo in his sleep wasn't the only solution to the problem, and a true Jedi would've tried all others first.
I dont really believe she would have done much flying other than moving it around or seeing her guardian mess around with it, so she would have no idea exactly what it can do. And the ship she originally focused on was most likely more reliable to use to get off the planet. Not to mention throughout the Star Wars movies the Falcon has explicitly been shown to, in fact, be a piece of junk (shown during ESB when Han can barely get the ship off Hoth and needs its hyperdrive repaired because it is so broken). Do you believe the ship itself is exceptional or just a well known hunk of junk that we like because it was flown by one of the series most beloved characters?
That was to be my next point. Even if she had flown the Falcon on occasion, it would've been just moving it around out in the open, not dogfighting and flying fast in very tight spaces. She wouldn't have had any idea how hard it could bank/climb/dive, yet she never catastrophically misjudged any of those variables even once during her escape. It's like how driving a Formula 1 race car to the grocery store a few times wouldn't prepare you for how it handles at top speed and with tight turns.
To answer your question, I don't think that it matters whether any of us think that the Falcon is exceptional (as Han does) or a hunk of junk (as the rest of the galaxy does). What matters is that, through four films, it's been proven to be a very capable and nimble ship.
