Geez. People are all "kill those thots" all the time here. I know this is a manga about killing unlikeable assholes, but up until now, if my memory serves me right, the MC only killed people trying to kill him and/or his companion. Her being a thot isn't reason enough to kill her. Now, her motivations for being there and what she wanted to do is enough reason. (There is magic and all and as killing is the MC only skill, you can safely assume, that any hostile character, that wants to kill them, can do so, if they aren't stopped (killed that is) beforehand.) The fine thing here is, that the thot's motivation might have been explained in the LN, but it hasn't in the manga here. I like that! The MC didn't know about her intentions as we readers also don't at this point. And so we've got us a nice moral question at hand.
The question is, is killing intent enough to judge, if someone actually tries to kill?
In other mangas MCs receive killing intents from their classmates or even from there childhood friends, just because a popular person said a nice thing to them. In some fighting mangas opponents ooze killing intent, because they take their opponents serious and without actually wanting to kill them(, but knowing that it might happen by accident, if things go wrong). Some MC release killing intent to sho away monsters or enemies.
But there are also settings, in which killing intent only means one thing. Getting ready for the kill.
Guess we can say it differs form story to story. I think we have to take the authors (unspoken) word for it. As eccentric the MC might be, he used his skill with good judgment so far (aside from experimenting with some victims). He judged the situation as dangerous. Also that world is fucking dangerous and the people are so unreasonable.
Therefore I personally will accept, that killing intent in this story means not only wanting to kill someone, but acctually being about to do so.
In this case someone lurking (being cloaked) and getting ready for the kill, while being far too strong to keep at bay any other way (she got magics) justifies a coup de grâce. Even if she was a cute twintail.
But stop that "kill her" thing, because a girl is a thot. Just hate her for it, as she deserves it. ?