King will notice that Zoro reminds him of another Samurai he’s faced in the past
This Samurai would turn out to be Shimotsuki Ushimaru whom King slays
King notices Zoro’s willingness and determination to fight even in extreme pain which is also a trait shared by ushimaru and their similarities in appearance. He would also mention their relation to Ryuma and claim Zoro has to die now
King would then confirm Zoro’s origins which Zoro at first doesn’t seem to care about yet claims he’d worry about that after or if he wins