I can't believe so many people missed the plot point

Kevin is NOT a 'psychopath without reason', the main cause of his issues is his ancestry.

His father is NOT his biological father, which is why he had no problem murdering him.

His mother's previous partner (an asian man), Kevin's biological father, is absent, which Kevin blames his mother for, the reason for his hatred toward her.

^
All of this is heavily suggested by the film. It gives good and clear meanings and reason, and quite frankly I'm stunned I'm the first person to explain this here, over 10 years from when the film first came out.

It is not a 'psycho for no reason!' crazy, simple film lol amazing how people really think it is, these same people shockingly go on to praise the film not even comprehending the real meaning lmao unbelievable