I'm actually a bit tired of the whole angel=good, demon=evil shorthand. It feels lazy now, especially in paranormal romance or romantasy. The most interesting stories flip it entirely. I loved how 'The Demon King' by Cinda Williams Chima didn't even bother with angels; it just made its demons a complex political faction. And in indie monster romance, you get 'demons' who are just misunderstood cinnamon rolls with a leathery wing aesthetic. The symbolism only works if the author does something fresh with it, otherwise it's just a visual cue I skim past.
Honestly, the wing descriptions themselves can be a dead giveaway. Pearlescent, glowing, feather-perfect wings vs. bat-like, tattered, obsidian ones. It's such an immediate moral billboard. I find myself more drawn to stories where the wings are ambiguous—maybe an angel's feathers are stained with soot, or a demon's leathery wings are surprisingly gentle and strong. That internal conflict written on the body is way more compelling than a simple alignment chart.