[ KNY Infinity Castle arc spoiler ]
Rereading the part where Kanao mocks and insults Douma and I finally understand that in a way, she's gloating.
Douma and Kanao were raised in an environment where neither had the luxury to express their true emotions. They needed to act in a certain way to survive. When they finally found a path that allowed them to be more of what they were, Kanao managed to shape herself according to her choice but Douma couldn't.
Even if Douma has gotten stronger, he would always be in that cult, always performing sweet smiles and playing the role of a savior. He stays in the way he was as a human. He's like a frozen statue, unchanging. His human self wanted a change, a spark of excitement in his dull life and he thought being a demon was the answer.
Kanao is everything that he's not. She finds the will to change while he doesn't. Douma has more power than ever, he could go find something else about himself but no, he let himself be rooted in the same spot.
" Why were you even born? " Kanao probably has asked the same thing about herself, and upon noting the slight similarity between her and Douma, she represented the same question for him too.
The brewing hatred in Kanao makes her express great pleasure in seeing that she succeeded in denying her past while Douma still lives in that bubble. Her words spit venom because she can relate to him. Can't feel happiness, can't feel sadness. While he can't, she couldn't.
The difference between them relies on their choices and she feels such immense delight in a moment of spite to know that what she chose has become her strength. She made the choice to change, even in slow progress, but she finds the will to live instead only to survive.
It's such a brilliant choice to have Kanao battles against Douma, from the fact that the people around them are connected to that their pasts are almost the same.
