Other mistake: When Luke is reunited with R2D2 aboard the Millennium Falcon, R2 plays the original hologram of Princess Leia from A New Hope. It is being displayed on the floor, to Luke's right. He tells R2-D2 that was a cheap move, and R2-D2 turns his head to look at Luke. Problem is, the device that he is using to project the image is on the part of his head that turns. We see the beam of light move when R2 moves his head, but when Luke turns his attention back to the hologram, it is still in the same place.
Revealing mistake: During the duel with the guards, one of the guards has a chance to hit Kylo Ren and Rey, but stops right before hitting them.
Continuity mistake: When Leia and Rey feel Luke's force fading, Rey has a dark, burned, non-bleeding wound on her right arm. After Kylo enters the abandoned mine, he sees Rey through the force. The wound from before is now bright red as if it were still fresh and bleeding.
Continuity mistake: When Rey meets Luke for the first time, the sun changes between shots.
Continuity mistake: While Rey is practising with the pole and the lightsaber, the sun changes between shots.
Answer: Kylo Ren has betrayed the First Order. They were loyal to the First Order. You are assuming they only did so out of fear of Snoke rather than out of loyalty.