So after coming across [this] video I actually sat and watched this particular fight several times. Kylo Ren is absolutely vicious during this fight, which is exactly why him walking away from Finn after striking him down (the first time) began to appear very odd to me. I mean, moments later we see him pressing his crossguard into Finn’s shoulder sadistically before slashing his back open. What gives? Why not just finish him off while he’s already down? And then something came to mind.

Rey? Hmm

Below is a crudely doodled step by step sequence of the fight I put together. The red is Kylo, the blue is Finn, and the orange is unconscious Rey.

Breakdown

The “come and get it” line is spoken, Finn is beside Rey while he challenges Kylo. 

Finn charges at Kylo straight on, they parry while Rey is several meters behind them.

Finn sidesteps, probably trying to find an opening on Kylo, but Kylo keeps up. This is when Kylo advances on Finn and strikes him down the first time.

Instead of killing Finn off, Kylo curiously walks away, towards– whoops Rey.

His advance towards Rey is halted when Finn gets up and challenges him again. Kylo turns around, his demeanour then turning simultaneously bored and even more vicious, this time really attacking him to get the job done.

During their fight the two switch positions before Kylo pushes Finn back against a tree, pressing his crossguard into Finn’s shoulder

Kylo and Finn are by the tree when Rey wakes up, Finn to her right and Kylo to her left.

Basically what I get from this is that the first time Kylo strikes Finn down to the ground and walks away, he’s more or less saying “stay down”. Probably assuming Finn will back down from the fight, Kylo turns around to go retrieve an unconscious Rey (who we already know he wants to take after Snoke ordered him to but would still retrieve even if he wasn’t ordered lbr), but his plans are disrupted when Finn stands up again.

Finn challenging him yet again gets Kylo pretty pissed. He dodges Finn’s attack and swings his lightsaber around in a bored way before wailing on Finn, pushing him against a tree and sadistically burning his shoulder.

Kylo Ren’s original plan:

Tactically knock Rey out before dealing with Finn

(*discovers Finn has the Skywalker lightsaber*)

Strike Finn down and have him back off. Traitor doesn’t want to get himself killed does he? Dude knows he can’t win. I’ll deal with him later.

With Finn down/too intimidated to continue, go and retrieve Rey

Pick her up

Force pull Skywalker lightsaber towards myself like a boss. Maybe kill Finn if I feel like it.

Dip outta here, I am not emotionally stable atm I need to sleep

What actually happened:

Finn gets up and challenges Kylo again despite the odds?? Holy shit

Why is this idiot fighting me I guess he wants to die huh

*cue painful shoulder burning and back slicing*

Might as well get the lightsaber now before getting Rey

*Rey wakes up and snags the lightsaber*

Well isn’t this just great 

Well that’s just what I understood from the scene of course (though the last bit was more about humour than legit analysis). This is how I saw the fight choreography and the character’s placements in relation to each other in the environment. I don’t think it’s so far-fetched for Kylo to underestimate Finn, thus walking away from the fight when he strikes Finn down once. He probably thought that was enough to get Finn to back off, so he was planning on retrieving Rey (what he’s really after – “we’re not done yet”) and then dealing with Finn accordingly if he had to.

What a one track mind.