How annoying that he has to reject Stella's offers to help. Didn't Yurisha tell him that asking for someone's help is another component of his fourth mission? I guess it's his stupid pride and all that. Accepting her help would've been the smart thing to do and might have ended the battle faster, now he has to face another pain because he wasted too much time lecturing his ex-disciple who's already beyond redemption.
I understand that him not killing is part of his character and a core message of this story, but man is it so unsatisfying to see him refuse to kill the guy who, y'know, unhesitatingly murdered him before. At least make an exception for this treacherous murderer. This outcome might've been the moral "right thing" to do, but TBH it just sucks as entertainment.