It's a moment that works particularly well for Illidan, it's perfectly in character and encapsulates what he is all about. Unironically one of my favourite WoW moments, cause it's earned. Long story short, (and somewhat inaccurate, but for the sake of brevity and convenience:) llidan sacrifices everything to obtain demonic powers that permanently change him, he does so in the pursuit of stopping the big bad while being shunned and exiled by everyone he holds dear. In this particular moment he gets a shot at "redemption" by this divine being, which begins to change him against his will. He proceeds to curbstomp said being while proclaiming that "his destiny is his own". He doesn't need to be purified because his actions are his own, the consequences are his to bear, his scars make him who he is and all that matters is that he sees his hunt to the end. It works because of his story, IMHO.
Now I didn't went on a WoW loresplaining spree with no intent, mind you: i just wanted to say that a similar arc for karlach would fall incredibly flat because her story so far is fundamentally different, she doesn't sacrifice anything of her own volition, she gets robbed of every single thing. However, in this imaginary scenario, it could be spun in a Vengeance/Forgiveness fashion. I personally wouldn't find it very cool, but it could work nonetheless. (Although yeah, I really don't know what asspull would be necessary to find something to justify a "Forgive and redeem Zariel" option.)
Now that you explained it, this sort of arc would fit Zariel if anything, not Karlach.
Still, I think that something on the lines of Karlach swallowing her hate could work to an extent. Another thing that gets only hinted at is that Karlach can be very selfish when it comes to saving her own skin. She mentions she would have not stuck out her neck for Wyll if he was in his place, and that she did not help the Elturans in Avernus. She also has no big moral issue using soul coins if it means she can win/survive. We know Avernus itself influences people and taints them, so I always thought this could be Karlach's "corruptible" aspect. Even Auntie Ethel's "custom" Vicious Mockery pull on that, saying that Karlach is not so wholesome when it comes to sacrificing herself. Some sort of story arc could have steemed from that.
