No he’s not 
He was Alliance until he took up Frostmourne, killed Muradin and betrayed his own people.
Then he became a Scourge villian, which is neither Horde nor Alliance. Also, Scourge != Forsaken. Forsaken are Scourge who, and this is the key part, broke from Arthas after regaining their free will and are no longer Scourge.
Just because Arthas is a villian to Lordaeron doesn’t make him Horde. Horde does not mean “literally anyone who’s enemies with Lordaeron”
“He was Alliance”
No he’s not
Arthas was never from Stormwind. Of course hes not part of the Alliance of Stormwind.
Fandral Staghelms main victims were Night Elves from the Alliance, making him an Alliance villain.
Archbishop Benedictus main victims were humans of Stormwind, making him an Alliance villain.
Lava Guard Gordoth main victims were orcs of Orgrimmar, making him a Horde villain.
High Inquisitor Whitemanes main victims were undead of Lordaeron, making her a Horde villain.
Arthas main victims were the people of Lordaeron and Quel’thalas, making him a Horde villain.
This isnt complicated
Dear lord no.
Just because he’s a villian to Lordaeron doesn’t make him Horde.
The definition of horde, AGAIN, isn’t “everyone against Lordaeron and Quel’thalas”
It’s a specific group of people. You can have more than one enemy at a time. Arthas was never Horde.
I already explained that like 10 posts up.
You dont have to be part of the Horde to be a Horde villain. Whitemane is a Horde villain. To be very specific, Whitemane is a villain TO the Horde.
When somebody thinks of Whitemane they dont think about Dwarves and Gnomes, they think about Lordaeron, because shes a Horde villain.
When somebody thinks of Magatha Grimtotem they dont think about Stormwind Humans, they think about Tauren, because shes a Horde villain.
When somebody thinks of Thermaplugg they dont think about Trolls, they think about Gnomes, because hes an Alliance villain.
When somebody thinks about Arthas they dont think about Night Elves, they think about Lordaeron, because hes a Horde villain
I think you’re either being trolled or dealing with someone living in their own world. Save your sanity and let them lol.
I’m suddenly reminded of all the times people have tried to pin Arthas as an Alliance character in an effort to paint the faction villainous.
Very funny to see this now.
Lol.
Perhaps even, dare I say, lmao.
… I’m sorry, what?
Last time I checked, Xal’atath is pretty openly hostile to both factions. She’s also technically undead, because, y’know… she’s infested a corpse.
Neither Lordaeron nor Silvermoon were with the Horde when they were attacked by Arthas, lol. You are really gonna die on this hill.
Oddly enough they’ve been better expansions. We’ve all seen when Horde get to drive the narrative.
I love these “Poor horde gets nothing” posts lol
They make no sense in any manner and they say it in full confidence lol
Dragon. She is not a Dracthyr.
She isn’t a void elf.
Villain.
Iconic alliance character before becoming a villain.
Only 2 are Horde really, 3 are alliance but the 3 alliance here are central to TWW and Sylvanas is just there for… Because she’s Sylvanas lol…
Horde is cool and doesn’t need fancy pop up halmark greetings on websites to remain so.
key art is auto garbage because alleria looks awful.
Arthas was never an Undead though ? He was never killed and risen.
Xal’atath is not a void elf
Horde are ugly, besides BEs who are fabulous. Only makes sense they would favor Alli.
The only difference is, even in those Horde driven past xpacs, the Alliance still got their own story to follow it. In these ones we’re just along for the Alliance ride.
I am more concerned about the fact that most of the shoulder armor is floating on most Horde races and several Alliance, like Worgen males. Nemesis in particular is quite glaring. I mean the original has the correct rigging and placement, why does the HD version hover? One thing the Devs absolutely need to do before releasing any armor is make sure the shoulder armor isn’t hovering on any race. I’ve done this work in the past on 3DS. It is not hard.
Alliance are the good guys, of course they are winning.