Even the High Elf Nagarythe people are considered to have a bit of a dark and violent disposition about them. I'm sure I read something about the land and it's people all being cursed by Aenarion's use of the Sword of Khaine, not just his blood line. With that in mind, if there were any Nagarythe elves loyal to the Phoenix King in the colonies they may have taken it badly when they were pretty much abandoned to the Dwarfs and would possibly then turn to the deep darkness within them.

If you just want a twist to your army background then I'd be cool with them being on the nastier side of Wood Elf behaviour (they are an army after all) and saying they are nasty because they are descendants of Nagarythe is feasible.