I don’t mean to excuse the stormtroopers - they’re soldiers for a genocidal fascist regime. But for some reason the movies go out of their way to mention multiple times that they’re recruited as children, and to demonstrate that, given the opportunity, many do defect, which I read as them being intentionally cast as at least slightly human or sympathetic (I assume the actual purpose of this writing decision was to justify Finn not requiring any redemption and that the wider implications weren’t really considered). It’s not like the First Order suddenly revealed themselves to be evil with the Hosnian Cataclysm but we’re still supposed to read Jannah and Finn as goodies because they eventually left.

Ultimately, I don’t actually have a problem with all the baddies exploding at the end of the movie - it’s a fun, melodramatic space opera, after all, not a hard-hitting gritty piece on the horrors of war - I just think it’d be less jarring for our heroes to be somewhat less utterly unphased by all the killing. It’s less about the stormtroopers and more about the way the heroes come across.

Edit:
Star Wars is very forthcoming with redemption for actions that would be less redeemable, or even irredeemable, in reality. Han Solo and Migs Mayfeld are both supposed to be sympathetic, despite being ex-Imperials, back in the era when you had to actually go and intentionally sign up to fight for fascism. Anakin/Vader has absurd amounts of innocent blood on his hands, but you’re still supposed to feel a little warm inside when his son smiles at his ghost. In TCW he’s supposed to be sympathetic and a goodie, despite having at that point already slaughtered a village full of humanoids out of rage. Bo-Katan is mostly treated as a good character these days, despite that time she helped burn down a village. We’re supposed to root for Ben Solo’s redemption even though he far more complicit in the Hosnian Cataclysm than any of the stormtroopers - he literally takes control of the First Order later in the same week anyway. Etc.