I think you're missing my point.
By killing someone with a force choke, he broke one rule. Saving Vader wasn't breaking any rules, he went against their wishes because it was the morally correct thing to do, but even then it wasn't a gaurentee. There's no morality involved in the act of having a child which is why bringing that up as a defense makes no sense. They're two entirely different situations, so it doesn't matter and is entirely irrelevant.
There's also the fact that in the original trilogy:
-Luke wasn't well trained
-The rules were barely established. At the time it likely didn't matter to Lucas and he put it in because he thought it was cool, not because he was consiously thinking about Luke using the dark side of the force. Things like that came after in the Prequels.
In what way does that destroy the argument? He didn't know about Rey, that's exactly the point. If she was his daughter, he would have, and she would have been enrolled in the academy because she's force sensitive. Something that manifests itself at a young age. I'm not saying that Luke knows of every force sensitive child in the galaxy and that they were all at his academy. I'm saying that it makes no sense, because he would have had a powerful connection to her because she's of his bloodline and a force sensitive. And before we bring up the "Well Leia, Luke and Vader didn't know they were family in the OT!" Luke's powers have grown far stronger since then, so that's irrelevant, and Leia, in the current canon can sense the force.
Anyway, if you're spending time studying a religion, and its commandments, you'd know the rules. You wouldn't need to "whip out a book" and even if he didn't, Obi Wan would've told him that "This is what led Vader to the dark side"