you clearly know nothing about wow’s lore LOL. Otherwise you’d never have said that. Before frostmourne arthas did do despicable acts, even tho it was necessary strathlomes purging damned him.
But the SECOND he picked up frostmourne it was no longer arthas, it was the soul of an orc named ner’zhul that controlled him. Much like how arthas controlled sylvannas when she first turned.
His mind and body did not belong to him anylonger.