Dogyo stole the Kusanagi no tsurugi because he thought that if the wonder-working of the sword became Silla's possessions, its divine power would help Silla to be a great power.
霊験あらたかな草薙剣を盗んで新羅の物にすれば、その神通力で新羅が強国になることができるというのが動機であった。