Mate, thats literally what happened, maybe watch the cinematic again, and see how his personality went back to how it was before he picked the sword.
Besides Arthas went there to avenge the people of Stratholme, but that clearly changed to him becoming a puppet and later the Lich King himself, that change started the moment he picked the blade.
this is the lore. you don’t have to agree. according to blizz, who writes the lore, you’re wrong though. 
Hmm? I’m confused did you mean me or him?
And why should i care about the writers whos recent lore largely consist of retcons?
Also that line can be interpreted in more than one way
i copied your post and it quoted you. sorry. lol
“it was still very much choices that he made that led him along the path that he went.”
read this part. very slowly.
Again can be interpreted in more than one way, for example, the choices he made which led him to pick Frostmourne.
Not every single choice he made after.
Arthas can killed Sylvanas again for us, for good this time.
That’s how he can atone.
the only people who would interpret it that way are completely delusional lich king apologists.
Arthas’ story is great as it is. I’d prefer his story remain ambiguous, or ended, as I believe a redemption could mess up his story, and I don’t think anyone could do it justice.
Sylvanus on the other hand, I don’t know if it matters as much. Regardless, I think they could make a small undead faction that allies with Sylvanus/Jailer and see what they could do with some story there. The fallout of Sylvanus’ actions could be pretty major, depending on how they decide to go with it (Night Elves and Undead) but I could see them glossing over it tbh
I thought it was her domination and overwhelming victory over the Horde that created the Sylvanas we have come to know
After the forests of Quel’Thalas mysteriously began to burn, Sylvanas and her rangers set out to discover the cause. She came across both her sisters being chased by a band of forest trolls, whom they dispatched quickly. Alleria alerted Sylvanas of the coming orcish Horde, and that it was they who were burning Quel’Thalas with dragon fire. Sylvanas and her rangers moved quickly to meet the Horde and trap them between themselves and the Alliance forces under the command of the paladin Turalyon. The battle took many turns but ultimately the Horde abandoned Quel’Thalas. Sylvanas stayed behind to hunt down any remaining orcs. Shortly after, the Horde was defeated, the Dark Portal destroyed, and the Second War ended.
Because he isnt the same bloody person anymore…
Picking Frostmourne turned him into a completely different person cause it removed the large majority of his humanity and twisted his mind the process.
you seem to think being a banshee is being the same bloody person. if you wanna talk lore we can talk about it. if you’re gonna keep crap posting knighting someone who actually tried to genocide an entire race (not a mass murder like sylvanas did to the nelves) while posting on a worgen of all things…yikes.
Ah, no worries! 
You really gonna bring the stupid furry hate when youre a Tauren?
Also do i need to remind you what Sylvanas did? caused a war with the intention to get as many people killed as possible, and is literally working with the master of death to kill everything.
“Redeem” Arthas by having him kill Sylvanus.
She’s become twice the villain he ever was.
i didn’t say anything about furry anything. worgen posters like nelf posters are known for being irrational regarding the burning of the tree. given the context of the post i made it is obvious you just pulled that out of your rear.
If you post with a Worgen you often get idiots hating cause “furry” or whatever.
You brought up the race, the connection is there.
no the connection was pulled out of your rear. if the first thing that comes into your head when someone mentions “worgen” is “furry” that’s an entirely you thing.
If Illidan can get a redemption arc, Sylvanas, Arthus and Garrosh all deserve one.
Garrosh did nothing wrong.
Sylvanas and Arthus are but minions of the jailor and if you disagree then the young boy-king is guilty.