Shinji and Asuka not taking joy in killing aliens is the biggest unrealistic part unfortunately; there's lots of real kids ganging up and murdering homeless people, and those are human beings. Real Asuka and Shinji would be holding the aliens in a chokehold while the other gleefully stabs it, both cracking jokes while they do it.

The adults do repeatedly send Kirito back into dangerous cyber-wars in the later books/seasons, it becomes his job because he's like the world's foremost cyber-melee-combat expert.

And in Naruto the adults send kids as young as 12 on dangerous military operations because once you get your headband you're pretty much considered an adult and a full soldier with all that entails. And this is a criticism, it's not an accidental worldbuilding artifact. Which makes it all the more frustrating that despite becoming a god and basically the ruler of the world, Naruto doesn't change this status quo.

Probably has a lot to do with it, yeah. I don't really watch shounen anymore. Kinda want to see if the new Jojo is any good, but that's about it. You just made me realise the only anime I've actively sort out recent years is Dungeon Meshi, so maybe I'm probably not the best to have an opinion on the trope lol

the logical conclusions

I watched a news segment about a kid taunting a burglar he shot (no not the onion skit) and I am now convinced there are many kids who'd get a chuckle out of killing people (and aliens); none of the adults around him were fazed by it, and the cops wanted to give him an award.

A real life Shinji would shoot an alien with his mecha and taunt it while it dies.