They don’t care that he was evil before wielding Frostmorne.
Arthas had a fetish for elven women. According to Edge of Night, he “violated” Sylvanas, and in the Sylvanas novel Arthas gave her body to an even bigger creep than himself. He was beyond sick.
I’m glad she was there in the end to smack his soul into oblivion; she totally deserved to be there to give him a farewell hawk tuah. Good riddance.