Meanwhile...
SN-2187 knows she screwed up. She didn't fire her blaster - she couldn't, not even as her friends died before her, not even as the villagers fought back - battle, even a slaughter like that, is nothing like the training sims -
Those people were not soldiers.
She should've fired her blaster to miss. She's vaguely aware some - maybe even many - troopers do that. A bit of a shameful piece of gossip, mostly overheard because not a lot of people like talking to SN-2187, especially not since she's been getting attention from higher up for her scores...
She's aware troopers who disobey so blatantly are reconditioned. She's aware Mar Ren would probably protect her at least somewhat - the Knight of Ren is mercurial, but seems to have an odd fondness for some of the troopers, and at the very least gets annoyed when Kylo Ren steals away her toys for his little pursuits -
She's aware what Mar Ren does with decommissioned troopers.
I won't kill for them, SN-2187 thinks. It's stupid. They say the Knights can pull the very thoughts out of your head. From how Mar Ren seems to be able to wriggle through any bluff, from how Kylo Ren always knows exactly when someone's politeness covers distaste or disrespect - well, it seems likely.
But SN-2187 is done.
Which means - if she's defecting, she might as well defect.
If she won't kill for them... Maybe she can stop them.
First - she needs the prisoner. The important one, who Kylo Ren personally dueled and took captive - surely high ranking in the New Republic or else the scattered Resistance bands that keep popping up in the territory controlled by the First Order -
The prisoner will be an asset, and possibly SN-2187's ticket into higher confidences. At least enough to get people to listen.
For getting the prisoner...
SN-2187 isn't a very good liar, but she's good at being believed, so long as she focuses well enough.
It's easy enough, to tell the guard outside the prisoner's room that Kylo Ren has demanded the prisoner's presence. It's easy, to go into the room and look at the prisoner and say, "You will be coming with me," as she manipulates the control's so the prisoner's shackles shift, binding her hands behind her back and detaching her from the chair.
(Other stormtroopers won't notice SN-2187's panic, at least not as unusual past the nerves of anyone interacting with Kylo Ren. Still, her heart's in her throat, her nervous anger the only shield she has wrapped around her mind.)