Which is worse, slaughtering eight people and leading three to join Hitler’s SS or slaughtering eleven people?
I think slaughtering everyone is markedly worse - rather that a fight breaking out in the midst of the confusion, right after Luke’s percieved betrayal of Ben, and him and the surviving students seeking the refuge of Snoke after the chaos, Luke’s percieved “betrayal” would be followed by Ben viciously taking revenge by single-handedly murdering everyone, that takes out any potential “gray” on Ben’s actions and the situation as a whole. In the other option Ben would not have killed everyone himself in a fit of rage, but rather taken a stance along those that sided in his favour after Luke “betrayed” him.
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As for Rey & Poe meeting, I can see the thought behind it. And yeah, they could’ve met at the end of TFA… but they didn’t. What I mean is there was no scene in which they met in TFA, apart for standing next to each other for a second. That seems like something that should’ve been shown, if it happened, but by the end they come apart without such a scene. So the conclusion is that they didn’t meet, not formally. They might’ve catched a glimpse of each other (which is why Poe answers he knows who Rey is) but that was it. And of course they’re apart throughout all of TLJ, so it feels fitting to remind the audience that despite having followed these two characters for two whole movies, they havent really had a chance to hang out with each other until now.
Also this might be reading too much into it, but to me this scene sort of feels like a bookend to Rey’s adventure and struggle in the film - she believes she’s no one, Kylo tells her she has no place, but then when she’s finally reunited with the Resistance the first thing she hears is “I know [who you are]”. And it pairs up so well with the reinstated deleted Poe/Finn scenes: “It’s fine, you’re alright, you’re here, with us, where you belong”.