Our hero, Finn, is a former stormtrooper who panics when he sees one of his buddies die in a firefight, the way a human being would. This affects him so much that, moments later, he decides not to murder a bunch of innocent people, the way human beings do every day. Then, the next day he kills a whole bunch of his former co-workers with a spaceship, the way the overwhelming majority of us never do ever.
I know it's weird to call out a movie hero for killing bad guys. Especially in Star Wars, where it seems like our heroes can't even walk down to Walgreens to buy some toothpaste and contact lens solution without making half a dozen stormwidows. But literally the only thing we know about Finn is that he doesn't like killing. And because he doesn't know his own name, that's the only thing he knows about himself, too. This is what makes us sympathize with him: "Death freaks this guy out?" we say to ourselves, "Why, death freaks me out too! Hey, this guy's all right!"
I guess you can't blame him. Stormtroopers have been wearing the same dehumanizing armor for two different empires now, even though it camouflages with nothing and is vulnerable to the primitive weaponry of tree-dwelling koala-cannibals. But hey, Star Wars: If you're going to design characters specifically so that they can be gunned down without a second thought, maybe don't tell us that they're all a bunch of kidnapped children.