Originally Posted by
Kyphael
A case can be made that he was as much the first victim of Frostmourne and The Lich King as his father was. He was in a no-win situation in Stratholme, and did what he thought was best. He ignored the mad-sounding ravings of an useless prophet (Medivh) and went after the perpetrator behind the crimes against his people. At this point yeah, he was a horrible Paladin, but not an evil person.
Goes to Northrend seeking out Mal'ganis, hears of Frostmourne, which can help kill Mal'ganis, and ignores his best friend's pleas. At this point he's beyond counsel, but still not evil.
Grabs Frostmourne and from this point forward, it could be argued Arthas' actions were no longer his own, as his soul was the first Frostmourne claimed, according to in-game lore. The biggest contradiction to this is Arthas; Rise of the Lich King where we learned Arthas was in fact in control after destroying Ner'zhul's very presence, but if logic was, Frostmourne took his soul, how could it be inside The Helm of Domination too? The lore isn't entirely clear, and every new tidbit of information contradicts some prior information.
His most evil act prior to picking up Frostmourne was killing the mercenaries.