Dr. Who wrote:
so the evil is not Arthas but the evil sword (well he was pretty bad ass but he add good intentions).
about Kil'jaeden i think i dont need to say nothing right? xD
sorry mate, but dont agree,
Arthas was more than a badass, he SLAUGHTERED the WHOLE Stratholme can u believe it ? He BURNED his OWN Navy, his stubbornness led the destruction of Lordaeron, Arthas (careful on here) made too much MISTAKES, he is the one who is responsible for the crimes in NorthEastern Kingdoms.
Kil'jaeden the Deceiver was once a pure Eredar, curious for knowledge. But one day, a titan called Sargeras offered an undeniable tender to him, endless power and eternal knowledge, exchange for his loyalty. Velen saw the trick and escaped, but Achimonde and Kil'jaeden became Sargeras' lieutenants. Sargeras was once The Defender of the Life and Protector of Pantheon, head of titans. Then he got corrupted and had a ill idea like "everything titans did is wrong, Titans are the source of chaos. To maintain order, i must DESTROY and rebuilt everything." and the created an army. Such and army that only leaves ashes behind. This army is called The Burning Legion, dedicated to perform Sargeras' orders at all cost. Kil'jaeden is now commanding it. He used Nerzhul to eradicate all resistance on Azeroth against the legion but his pawn turned against him.
Kiljaeden tortured Nerzhul in twisting nether endlessly and then imprisoned his soul to a sealed Throne, (as u know Frozen throne) and commited to give him a healthy body if he obeys his orders, but Nerzhul swayed Arthas' mind (it was arthas' fault) and gained Arthas' body; became the most powerful being ever in Azeroth. It is still a speculation that does arthas possess Nerzhul's power and rule the Scourge or did Nerzhul capture Arthas' body;
NOBODY is evil, everybody wants gratification and makes mistakes. regardless of the answer, scourge is the major threat and we all must focus this fight, if we want to live.