@icarushector: I get a feeling that the exorcised girl either didn't intend to hurt that person, was framed for it, or attacked because the person insulted her first.
Because Evan only told the story briefly, we didn't get a full details of things, but imagine something like:
Today is the day she know she'll celebrate Christmas with him (since he got a plush and food ready) and she's super elated.
In case of the accident attack: Well, demons are stronger, so if she was just 'dancing in joy' without looking and a passerby is just in her way...well...oof.
Framed (the most likely, given how she knelt down + his mention of her siding with demons): Some demon came in, tell her to stop trying to befriend their enemies, etc etc, she refused, they attacked a person, and while the girl was checking the victim's condition, Evan shows up with a look that basically told her what he thought. Despaired that his first reaction is NOT to trust her innocence despite this near a year of interaction and thought that he's been just pretending to go along with her the whole time (in reality he is opening up to her, christmas gift/dinner and all that), she kept quiet or assumed guilt and let him exorcise her.
Mocked: The victim, a woman with a crush on Evan, has been seeing this tiny demon clinging to him and got jealous, confronted her about it. Something about he's just playing with her and he'll NEVER like a demon is said, demon girl attacked, Evan show up, regret and despair, etc.
Whatever the case, her reaction at the end probably clued Evans in that he had prejudged her and that's the thing that hurt her the most.
That realization made him go into self-loathing mode over how he abused her affections for him, then our demon-chan shows up, and he thought "THIS TIME, I won't repeat my mistakes"
tl;dr - Author can still use this for more FEELS chapters.