Never trust a knife ear.
Blood Elves were based on a STRONG request from a poll of Asian players where many remarked on the Horde side that they and their girlfriends wanted a non-creepy femme race to play (Source)
I actually have played Horde in the past, so this point doesn't matter. I also have a lvl 120 BE paladin, while no void elves. It's just my opinion as a person, and not as an Alliance player, that Lor'themar is absolute garbage nobody who has never done anything, other Blood Elf characters (Kael'thas, Liadrin, Rommath) are way more recognisable and relevant than he's ever been. He's blindly following every genocidal filth of a Warchief despite being a proud Sin'dorei. I mean for real, no hate, but he is just... nobody. Even Gelbin had more impact than him (Even Baine).
"We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see." ― Tom Stoppard, Rosencrantz and Guildenstern are Dead
Nathanos refers to them as Lor'themar's rangers, not Sylvanas' - I don't think the Dark Rangers would take too kindly to that if it were just them. You can also see NPC's running during the fighting itself which are tagged as "High Priest/Priestess of Silvermoon."
"We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see." ― Tom Stoppard, Rosencrantz and Guildenstern are Dead
That's not what traitor means. It's entirely unreasonable to believe that he intentionally exiled the elves meddling with the void so that they would join with the Alliance and the Horde would lose future wars. He stands to gain nothing.
It may have been a short-sighted decision given that we know with hindsight that they joined the Alliance, but that couldn't necessarily have been foreseen. There are many independent organisations and communities on Azeroth, and I believe that the exile of the Void Elves takes place long before war became inevitable. The Alliance is not exactly known for being accepting of Shadow, it's not very on-brand for them. Also choosing executing people for forbidden magic over exiling them would be a pretty bad look for the Blood Elves given their history.
It kinda evens out. The nelves exiled the belves for the use of magic, belves went Horde. Now some of them return to Alliance. Well.
Oddly, none of the Blood Elven units (except Lor'themar himself) are in the red/green/gold finery you'd expect - all of them wear more subdued colors such dark blues, grey, and white instead. Not sure if they're looking to somehow camouflage themselves, or if it was just a stylistic choice.
"We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see." ― Tom Stoppard, Rosencrantz and Guildenstern are Dead