i think lee rang was more upset by the fact that lee yeon accepted the fact that he abandoned his younger brother moreso than that he abandoned him, he probably wanted him to deny it just look at his face.
this is my theory after i read the character description that was released before totnt started airing but i think lee rang has a sort of internal hatred for his human side since he isn't pure fox. he probably mainly admired his brother because he was a forest god pure fox which is why he was more devastated when his brother abandoned him and his position for a "mere" human and his aggression towards humans increased. (side note kim bums acting here 👌🏻👌🏻 idk how actors can instantly make their eyes red and then make it disappear, i love the small details he puts into his acting that normally would go unnoticed)
and not to say that its okay what he did to the humans but from what i see he uses a kind of demented view of right and wrong and kind of "fufills" humans wishes and punishes evil doers. (dont mention jia for this example because he left only scratches on her and didnt eat her even if he threatened to thats different from him actually doing so and all the other questionable ones are unknown ie the girl from the bus) again this is not to say its okay that he does this but that it isnt as bad as some other villains from other dramas or the one that will appear in the future in this one as well bc rang is most likely not the final boss.
[first gif is from yesdramas and the second is from holywitchlady]











