This is an interesting reading! I really love how you are extrapolating on Kylo’s character in terms of his combat style. Personally, I think an examination of this fight in context shows that it’s definitely not senseless brutality.
If anything, this gesture is evidence of Kylo’s mercy. If you have a deadly laser sword in one hand, and the other hand is a fist, and you chose to subdue your enemy with your fist, doesn’t that imply that you don’t want to kill him? Kylo incapacitates Finn in the quickest and least violent way possible. His fighting style is graceful and flourished, yes, but that’s consistent with the forms he’s drawing on, and probably had (limited) training in as Luke’s pupil.
While Kylo’s cry of “TRAITOR!” is intensely emotional, I think it’s important to consider that he is an emotional person reacting to a string of personal failures and the death of his father at his own hand. This obviously informs his behavior. He may be attempting to redirect his rage (at himself, for failing) toward Finn, holding Finn responsible for the events which just transpired. Redirection of aggression is a common dark side tactic, after all, and likely something that Snoke has coached him in.
Because Finn is now unarmed, Kylo is specifically electing to minimize use of his weapon. Keep in mind that, even though Finn is unarmed at this point, he is still dangerous to Kylo. Also, there’s a context, here: Finn has just lead an attack on Kylo’s base and left carrying something that belongs to Kylo. Finn came to Kylo’s proverbial house, took one of his found-family members prisoner and threw her in a trash compactor, helped install a bunch of explosives,
and then made off with grandpa’s antique sword and an important map (that Kylo absolutely needed.) In context of Kylo’s needs, motivations, and psychology, it becomes clear that this is definitely a defensive action – not some exercise in savagery.
Actually, all things considered, I think it says a lot about Kylo that Finn survived this fight. Definitely an interesting and important detail.
The Archery Range 