one day

tae su is a hurt character, if you watched the movie you'll see that he always asked his family why they should be sorry for him, since they never accepted him , and were always using him, even tho he kept forgiving them and expecting love from them, that way he was never thaught how to feel or express his feeling because of his adoptif parents' neglect and abuse and never learnt how to love and always thought that he could show his love by killing people since that is what his brother wanted from him,and you can see it when he asked the little girl if she wanted him to kill the happy family. so he build borderline and abondenment disorders and his envy of other happy families transformated to hatred, to the point that his life felt meaningless unless he feeds on his grudge of the unwelcoming world, he only wanted one thing and that is a loving and accepting family and a mother's love.