Another isekai, another unfulfilled fantasy... Oh how I dream of the ultimate sword. You pull it out of its sheath, and reveals itself as a gundam on a handle. You would swing the gundam around, and the gundam would swing a little sword of its own, and shoot lasers while its eyes glow brightly.
On a more serious note, while it makes sense to murder an incapacitated and captured ally to prevent them from revealing any information, this was not what happened here. I'm fairly confident this guy would not have talked. More importantly, she brazenly revealed her position in order to needlessly kill an ally that could still be used as a distraction. Of course, I understand the trope of bad guys murdering their allies (especially in this case where they evidently didn't get along) the instant they are deemed useless, but the way this was done here just seems very stupid. Of course, our sword saint had to go and reveal his sword's ability to his opponent after having defeated him, so clearly not a bright fellow either.