Just watched this flick, not sure how I've missed it all these years. This is definitely an art house type film shown in a non linear fashion, so it won't be up everyone's alley but I like how it was told.
Now I have to correct some of these other reviews I've seen. As someone who works in the psych field and deals with patients from toddlers to seniors, I'm putting it out there right now that Kevin is not a sociopath but straight up a psychopath...and yes, they are different. Officially he can't be diagnosed under 18 with antisocial personality, so he'd be under some conduct or oppositional defiance disorder until he's 18.
Psychopaths are usually born this way as oppose to sociopaths that are created through childhood trauma. Sociopaths have little empathy but psychopaths have zero empathy and cannot form true bonds with others. Their bonds are strictly superficial and self serving. They are highly cunning, calm, collected, and manipulative which can lead them to be very charming (think Bundy). Sociopaths are more impulsive, erratic, and have angry outbursts. Being a psychopath is how Kevin was able to manipulate everyone into thinking he was a good kid, all the while messing with his mom's head cause he enjoyed watching her squirm.
I've also seen other posters blame Tilda as the mom, saying she never bonded with him. Well, that may make him dislike her, but that won't turn him into a killer. He's a born psychopath, period.
