All Stormtroopers are taught from an early age to follow orders without question. So why did Finn betray the First Order?
Finn chooses to betray the First Order because he doesn’t want to participate in their atrocities. He refuses to take part in massacring a group of innocent villagers, realizing that he needs to escape the First Order before he also commits a horrible crime.
He is able to help a Resistance pilot escape the Starkiller Base and free himself from the Order. Finn has no emotional connection to the First Order since he is kidnapped as a child and forced to join.
Finn decides to betray the First Order
At the start of Star Wars: The Force Awakens (Episode VII), a group of Stormtroopers attempt to steal a map leading to Luke Skywalker. Finn, then known as a generic trooper, shows care and sadness when a fellow trooper passes away.
Stormtroopers are kidnapped from their family at birth and raised to fight with no conscience. Witnessing the trooper perish, Finn stands up and begins to panic. He looks around at the crimes and atrocities his fellow Stormtroopers are committing on innocent civilians.
Kylo Ren, leader of the First Order, lands on the scene. He quickly questions Poe Dameron, a Resistance pilot who successfully hides the map to Skywalker on the droid BB-8. Ren motions to place Dameron on the ship. Ren is also questioned about his family and his potential connection to the light side of the Force.
Captain Phasma, a high-ranking Stormtrooper, asks Ren what to do with the villagers. Ren says to exterminate them all.
As the Stormtroopers raise their weapons, Finn hesitates. He seems genuinely disturbed by executing the villagers, and merely pretends to fire his weapon as the other Stormtroopers do the dirty work.
In shame of what he’s witnessed, Finn lowers his weapon completely. As Ren walks back to the ship, he stares at Finn, seemingly confused over the Stormtrooper’s lack of participation in the massacre. Nevertheless, Ren continues on to the ship.
So that’s the answer. Finn decides to betray the First Order because he doesn’t want to participate in their atrocities. He sees them massacre a group of innocent villagers, which sparks the realization that he needs to run away as fast as he can.