He wasn't strong enough~

I know he has a heart, regardless of what he may seem like on the outside. He would show him mercy with that golden heart. During the fight with Tsubaki and Masamune, he had to wait for her outside of the sword. He was being simultaneously hit in the head with rocks for no good reason. You would expect him to blow up at the people doing it, or get out of there, but nope. He waited for her, no matter the pain. At that rate, he would have been able to defeat Mifune, but I know for a fact that he would most likely spare Mifune.