Can someone explain why he was smirking at the end it gave me the chills

I think it’s because he was playing him and never really liked him... and now he’s gotten him turned into an omega and that will probably ruin his life.. and now he’s attached to him forever

hes just a person who has a twisted kind of love, hes been obsessed with him for years and now he’s finally able to obtain him

i think it's because he does like him and that's why he lured all Tsukasa's exes away. he is smirking in the end because he finally got him, though it's not love but extreme possessiveness.