Finn has this sudden awakening to the horrors of violence and leaves the FO… Why? Why now?? But, whatever. Fine. Ok. He just 'does.'
But in the process of escape, he's maiming and killing LOTS of FO soldiers. People he knew. People he knew had been abducted as children and brainwashed to serve. He goes as far as skewering one right through the chest with the lightsaber. And throughout the violence, Finn is all 'Woo hoo! High fives!' This makes zero sense.
It's a strange gap in his character. It was kind of okay when they were clones… Except, no. Still human. And kind of okay when they were recruits because then they chose to serve evil. Except no, cos maybe they don't understand the Empire as evil.
But now that we know FO Stormtroopers are all presumably abducted, brainwashed children (Sheesh, Disney… 'Go dark or go home, hey?) – and no one understands this better than Finn - it's very strange that he's not only not refusing to kill them, he's not even a little reluctant or hesitant. Not only is he not hurting with Kylo Ren-sized angst when he has to strike a brother down – not only is he willing – but he seems to actively enjoy it, like a game.
They need to deal with that. My guess/hope is that it's something like 'in combat situations when he's in actual danger, his programming just kicks in' or something. It's fixable, but needs fixed. This explanation is a bit weak and unoriginal, I know, but it could work. It's at least better than nothing. I hope we're not expected to keep watching this guy who's 'only here cos he's anti-violence' keep literally burning holes through people without any sense of conflict or explanation.
What I'm really hoping is that we see some of the violent action from TFA from Finn's POV as he's in the coma, the trooper he literally ripped through being the most haunting vision. Maz, Luke, or Rey helps him process this. It's hard and scary. But he does it. This not only brings him to terms with what he's done, it also breaks his Stormtrooper programming. And then he wakes up.
But who knows what will actually happen? They just really need to directly address it. It's a huge writing flaw right now.