One of the short stories goes into Finn's background and mentality a bit more, but everything you need to understand him is right there in the movie.
He's not a believer in the First Order. Never was. Up until very recently, he'd never really questioned his place in the galaxy in any depth. He was a Stormtrooper, which to him means training, routine, discipline and team work. That was his assigned role and for the most part he accepted it. Jakku was his first live fire engagement and it was horrifying.
His compatriot dying right in front of him (and for what!?) was the first time he was ever truly cognisant. It snapped him out of his unthinking state and into blinding awareness of the scene surrounding him. Indeed the magnitude of his distress was so great it sent a ripple through the force that even Kylo could sense. This is his first turning point.
As for later turning on and even killing his former compatriots; as other have said this is his second turning point. Where he overcomes his fear and shame and chooses to fight in the defence of others in the name of what he feels is right. Something he never felt as a Stormtrooper.
One of the key traits to understanding Finn is his basic need to be helpful. To work as part of a team. It's what made him such a good trooper in the simulations and we see that trait is still present even while he's still running. First with Poe, then with Rey and even later with the pirate crew where he seems positively eager to help them load their ship.