Skoll Shorties

After the Third War Sylvanas basically broke free, broke any of the Scourge free and turned them on Arthas. Her actions in Classic to Wrath while grey were not too irredeemable and at least it was out of a just goal. But that goes down hill after Arthas is dealt with. --

She's no longer under anyone's control and her new reason for existing and "thriving" is to not go to Hell. Her goal of revenge turns into a goal of survival. And a very selfish one at that drives her to do very bad things. Far worse than what Arthas has ever done. --

Going back to Arthas, yes he did terrible deeds but most of them were under the influence of Frostmourne and the Helm which (spoilers) are now revealed to be linked to the Jailer. Sylvanas doesn't really have that excuse anymore which is why "redemption" doesn't feel right.

I think the problem that most people have is all of the things LEADING up to Frostmourne. I think that's the point no return ,but he had done a number of things call his morals into question before that (stratholme, the burning of the ships in the northrend expedition)

I think that we've got characters who have done far worse while in control of their actions though. Stratholme wasn't out of malice but out of stopping something horrible. Northrend is when things go south for Arthas that I'll agree.

- Killing the Mercenaries who he allied with - Burning the Ships of his Men - Lying to his men - Leaving Muradin behind Those are the crimes he did without Frostmourne and yes they're extremely messed up. But I don't know if he's irredeemable at that point. --

On its own, none of those things are irredeemable, but it calls into question just how corrupted he is once he takes up Frostmourne. Yes we know it takes his soul, given what he did to get it, but how far does it move the needle when he takes up the blade?

That really depends on whether you think Arthas picking up Frostmourne is still Arthas. To me that's Arthas possessed and taken. He's basically at the same state that DW is when DW was controlled by the Old Gods.