The wife of Dronacarya, Krpi, is the sister of Krpacarya. A devoted wife, who is according to revealed scripture the better half of her husband, is justified in embracing voluntary death along with her husband if she is without issue. But in the case of the wife of Dronacarya, she did not undergo such a trial because she had her son, the representative of her husband. A widow is a widow only in name if there is a son of her husband existing. So in either case Asvatthama was the representative of Dronacarya, and therefore killing Asvatthama would be like killing Dronacarya. That was the argument of Draupadi against the killing of Asvatthama.