Sacrificing oneself for their people is not mutually exclusive from being a terrible person. Merle Dixon sacrificed himself to save his brother and his friends, that doesn’t make him a good person. And despite Anakin seemingly getting to be a Force ghost for saving Luke, he’s still a mass child murderer.
Similar to what we we saw in Thrall: Twilight of the Aspects, if the Legion hadn’t sent the Scourge to Azeroth, Arthas would have gone on to marry Jaina and wouldn’t have purged Stratholme or done anything particularly grievous. But we would still know that this alternate Arthas was fully capable of being what our universe Arthas was.
Just because Arthas gave Sylvanas a reason to do what she did doesn’t mean she wasn’t always capable of it.