I think it’s less that “it’s the first time you’ve said the word” and more “it’s the first time you've used that word in this context”. Takahashi is trying to get Itou to understand that Takahashi and N are not really any different from each other in the great scheme of things. How is N’s actions with Hiraizumi any different from Takahashi’s? Is it only because Itou has formed a relationship to Hiraizumi? Does that mean that Takahashi’s previous inhabitant was less than Hiraizumi? Because if N’s actions are blasphemous, and he should be destroyed, then Takahashi should be destroyed as well because her actions were no different, regardless of the context of whether one was suicidal or near death or what have you.
If it is the piloting of a body and deceiving those who care for it that is the blasphemous act, the context of the puppeting is meaningless.
what's interesting to me is that there's cold logic to Takahashi's thought process here, given that she's inhuman just like N is.
But I have to wonder whether Itou doesn't see it that way, by virtue of the bonds she specifically has formed with Takahashi, and the sum experience she's internalized where Takahashi has been a friend, confidante, ally, and yuri-panion (meta call). She doesn't have that with N or the new monster wearing Yoru's skin. And at this point, whose interpretation of the "good" and the "evil" thing is correct, or should be considered correct?
Does Itou's feelings about Takahashi have weight? Do
our feelings as readers of this story, and our meta-knowledge of the characters' various positions and roles within the narrative, have weight?
And why should credence be given to some over others, if at all?
Which I think circles back to the heart of the matter of both times Itou used "blasphemous". Because both times, it was Itou rejecting the cold logic that Takahashi was imparting to her, on the basis that the emotional tax she would suffer in following that advice and how she felt it wasn't right or correct or sound to remove some innate part of herself in order to "fit into" the sort of mindset required for such dispassionate thought and behavior.
There's a real irrationality in wanting to destroy the monster taking over the body of a former friend/classmate, when you're the only one who knows they've been killed and puppeteered. Takahashi is absolutely right: Yoru's friends and family would be devastated if she suddenly were truly dead. But Itou can't let the monster go on living Yoru's lie, because to do so would be a rejection of her memory of Yoru. There's a humanistic element to not wanting Yoru being desecrated like that.
Just like "being kind to others means being transactional with others" was Takahashi's advice for how to better make friends, and yet Itou felt that doing so would be lying to herself and to those she wanted to get closer to. It's rational, or pragmatic, to think about friendships as "give and take" and to reduce it to unfeeling calculus and rates of exchange, but you lose something very human in the process (call it your heart, or soul, or whatever works best).
Apply the irrationality of not wanting to remove the human heart from the equation that Itou has embodied twice now, and I suspect she would find a way to similarly rationalize to herself why Takahashi needn't die, like she desires N & Fake Yoru to die, solely because of the emotional bonds she's built between them (at least from Itou's side, but I'd argue from both).
And I suspect that is where Itou can teach Takahashi, for once.