Apparently some people think Arthas was the most evil character in the story. Yes, he did plenty of fucked up stuff, but chronicle 3 established a noble goal for his actions.
"A world ruled by the undead would have no more injustice, no more wars, no more mortal flaws. Perhaps most importantly to the Lich King, he believed his Scourge would be far more capable of defending Azeroth against threats that would try to conquer it. He had observed the awakening of C'Thun and the Burning Legion's attempts to launch other attacks on Azeroth. Neither the demons nor the powers of the Void would rest until they controlled the world. A fractured world, constantly beset by skirmishes between the Alliance and the Horde, simply would not be prepared for another incursion." - Warcraft Chronicle vol. 3, page 172
If people can justify Slyvanus' betrayal and atrocities by believing/hoping she is doing them for some greater good rather than out of selfish fear for her own judgement in the afterlife, making her worthy of redemption, why can't Arthas also be redeemed as well? His actions in Stratholme weren't for the sake of being evil, he thought it was the only way to stop the spread of undeath. If Slyvanus' mass murder of the Night elves can be "justified" by her end goal to "free" us, shouldn't Arthas' mass murder of the High/Blood elves also be "justified" to combat the Legion and the Void?