#semi-related: i think fandom throws around terms like 'war crime' etc too flippantly at times?

[undercat-overdog]

"It's ok to kill elves because they respawn" and "death isn't serious for elves" will never not be a hilariously bad argument.

Respawn is hardly guaranteed. At least some individuals (Aegnor, Feanor, Finwe) will not be reborn, at least for the entire lifespan of the planet; death can be permanent. I think the various writings about Mandos and rebirth (plenty of juicy stuff in NoME in particular if you're looking) imply that it's not particularly uncommon for Elves not to return to life.

Mandos isn't a pleasant place to be nor is it where Elves are meant to be. Its name literally means "prison" - the word is related to "Angband" even (the "mandos" and "band" elements are cognate).

Death itself is pretty damn traumatic! Even if I pop back up in 15 minutes like a mob in an mmo, a sword through the gut's not fun.

All evidence suggests that Elves (and Valar) treat death very seriously and don't think it's unimportant. The Elves of Beleriand don't even sing songs about Fingolfin's death because the grief is too much.

For Elves of Middle-earth who are killed, go Mandos, and are reborn in Aman - they are sent somewhere that they may not willingly have chosen to go and can't leave. There's not really a great real world analogue to this (I'm intentionally avoiding the potential closest ones for reasons) but I don't think it's good and removing that choice - through violently murdering someone - is itself a crime.