The sad, quiet little look Kylo Ren and Finn exchange at the beginning of TFA is one of the most important points of the film. But it goes mostly unnoticed by fans and critics.
Kylo knows, in that moment, that Finn can’t be trusted. That’s he’s not a killer. And Kylo tells Hux later, with 100% accuracy, that he knows fine well Finn was the stormtrooper who helped Poe escape.
So why didn’t Kylo say anything at the Jakku massacre? Why not just kill Finn on the spot? Or tell on him to Phasma and Hux?
Because that last little bit of humanity he had got to him. He let it slide (temporarily). He let Finn go.
But it’s not just about Kylo, it’s about Finn: Because he looked right back at that guy and realized: “Well, I’m not a hero, but I’m not that, either.” It sets up Finn’s character motivation in the movie.
Seriously: Watch that scene again. It’s rather crucial to understanding the whole thing.