This is a theory, that is unlikely, but would make Kurou a better guy than this tends to make us think.
To my mind, Kuro knew the original Touka (before she died en got remplaced by "someone else using Hirasaka Touka's body", in the following, I will say Touka for the original and Shrine Maiden for the one who is using the body ((??Touka?? could work too)) ).
First possibility :
At that time Touka was in a near death situation, she either has a weak constitution, or has an incurable disease, or whatever, and Kurou wanted to save her, and that is the reson why he became doctor (at that time, or later, 'cauz we don't her condition at that time, when it did start and the age of Kurou at that time). In the end, medecine was unable to save her, and decided to make a ressurection ritual.
Kurou has either attempted the ressurection, while sacrifying her mother, which ressurected instead of Touka (but, she would probably remember him if it was the case), and got a video.
Second one :
I think that the Shrine Maiden, make children, and take their place when they are old enough, he might felt that something was fishy after speaking with Touka, wanted to protect her, but was too yound and coun't do much, sneaked in during the time of the ceremony, and got a video.
He studied Medecine, trying to understand what coud possibly have happend during that time, and couldn't understan
He wanted to know what did happend to the Touka he knew (may have a thing for her), and he is the one, who is making people come in the island, through the web, using them as pawn, to learn more about the mecanism of ressurection. That would explain his gloomy side, which make him a good match with the schemer and makes him a proper villain : someone which could have been the main character of the story, told differently.