User blog comment:Alockwood1/Sir Daniel Fortesque Versus Samurai Jack/@comment-5043545-20150724003620/@comment-5043545-20150724025616

While the whole purity thing may be true, the problem is the situation of Sir Dan. He was resurrected by an evil wizard using necromancy, easily one of the most evil forms of magic. Aku, himself has used necromancy before, Jack has slayed beings created by said evil magic. Meaning that Sir Dan falls underneath non-purity part as Dan has been tainted by death and necromancy. If you need proof then go to episode XXX: Jack and the Zombies. The sword is fully capable of harming Sir Dan.