Fighting casually is a great way to lord your dominance over lesser combatants. You're so good that you don't even need to put your full effort into defeating them. Just don't get cocky, kid.
Commentary by memnarch (who has not seen the movie)
Finn held out better than I thought he would! And he also seems to have avoided bisecting himself with that blow as well; a bonus!
Sally plays Kylo as a pretty good villain here. It's similar to the odd "liking everything bad" line that he'd said before, but here it's presented as being a very ego-centered bad boss, so it works a lot better.
And yeah, it's pretty obvious here Kylo isn't taking this seriously. One-handing a lightsaber while fighting just about the minimum possible while still actually fighting back.
Finn: That’s why I asked Phasma for a spy mission. To find Gil’s killer.
Kylo Ren: And did you?
Finn: I did! That Resistance pilot you turned into a spy!
Kylo Ren: You killed him, then?
Finn: You shot us down! He died. I barely escaped.
Kylo Ren: So. I exacted your revenge for you. You should be grateful!
Kylo Ren: I’ve been fighting casual so far. And now... oh, by the way... What was your name again?
Kylo Ren: And now, Finn... You will die!
GM: That blow knocks Finn to the snowy ground.
Kylo Ren: The sword hit or the sick burn?