You are very correct but I think this is more-so showing a potential crack in the emotional wall he's put up. Its a small admission to himself that he is starting to like her now if only a little.
I have been waiting for the author to dig into what exactly happened in the first place (morbid curiosity I guess idk) but if this is Acid Girl then it either might have either been an accident, she might be regretful, or maybe she was the original intended victim and Mikoto took the acid for...