I want to start off by saying I like Finn's character and he is my favorite newly introduced character in the sequels, so in no way do I hate him. There is, however, a problem I had with the way he was introduced. I personally thought the concept of someone being in the First Order their entire life and turning to the Reistance was very cool. I did feel like it was executed poorly though.
First off, we see Finn's friend shot and killed in the opening scene of The Force Awakens. He was very close friends with this guy and he always stuck up with him which hurt his reputation. It's pretty logical Finn would be furious someone from the opposite side (Poe) shot his only friend and defending a clumsy trooper for so long was now for nothing. Obviously Finn should be outraged!
Instead, he helps the same guy who shot his only friend out of a cell where he had no chance of escaping alone! Wouldn't Finn have captured or even killed Poe instead of helping him? It would have made much more sense.
Then he goes from being traumatized from his first mission to unhesitantly killing unarmed engineers he may have grown up with in defenless towers in minutes!
I maybe missing a key factor here, so if love if someone can tell me why I'm wrong.