My theory is that, the episode in Kevin's life where he was left at home and potentially killed by the then "Wet Bandits" (had it not been for the guy with the shovel) brought really severe negative consequences to his well being, and he was damaged psychologically and scarred for life. Here it is why:
After the family came back from Paris, and they all had their hugs, kisses, and Merry Christmas', Kevin apologized to his family and confessed that he was the one that made them disappear. Knowing what we know about the McCallister Family, they probably laughed at him, called him all sort of names, and the parents told him about them sleeping in and ultimately forgetting him in the attic.
After realizing that his wish had not ben truly granted, and that his parents were just careless, he got mad... he got REALLY mad. Now he knew that there was no Santa who made his family disappear; no Santa who made his family come back. Christmas was just a big, fat lie.
He spent the rest of the next year resenting his family, and, as we find out in HA2:LINY, his family situation had not gotten any better: He is still bullied by his big brother Buzz and the rest of his family. During the Christmas concert, Buzz makes him look like a fool in front of friends and families, and somehow, Kevin ends up paying for it, being literally JUDGED by the whole family.
He is sent to the attic, ONCE AGAIN, the night before their big trip. Obviously the parents are not afraid of something happening again, after all, what are the odds, right? On their discussion, Kevin's mother even suggests: "be careful, or you may get your wish granted again", and Kevin's response, very appropriate for a pissed off 9 year old, is "I sure hope so".
So, dad disconnects the clock, they over sleep, yada yada yada... They only realize that Kevin was left in the attic again when they are in the van ready to leave, but surprise surprise! Kevin is in the vehicle already. So, everybody has taken a seat in the van, all seven of them, and nobody noticed that Kevin had been sitting in the front seat the whole time. Pretty shitty, if you ask me.
They get to the airport, and what does dad do? looses Kevin out of sight. He takes off running like a maniac! O'Hare airport, December 23rd???? what kind of parent would even CONSIDER letting go of his youngest child's hand and takes off running? Pretty shitty, if you ask me.
Kevin knows that his family just does NOT care for him, and that obviously the previous experience didn't mean anything to them. Christmas has become a time of hating, a time of lying, a time of realizing that he means nothing to his family. He has a lot of anger inside. He officially HATES CHRISTMAS!!! and everything it represents.
When he is in New York, he visits a big toy store; what does he sees inside? toys, santas, happiness, families, dads playing with their kids, moms buying presents, all things that he is missing from his life. How does Kevin shows his disgust towards the toy store? goes and breaks the front window with a brick. "Well, he broke it so the alarm could go off", you may say. To which I will respond: "Well, he had the tape where he recorded Marv, the goofy criminal, telling him in detail how they were going to steal the money from the toy store. Why in the HELL he didn't just go to the police and turned the tape in, along with telling the police that the two criminals had just escaped from jail?"
Which leads me to my next point: All the torture, sadistic, and unnecessary pain that he put the stick bandits through was completely and absolutely unnecessary. Kevin gets his fix in inflicting suffering to others. The situation with his family has pushed him to have no sympathy or remorse towards others.
Throughout the move climax, where he puts the sticky bandits through all those "HOSTEL' and "SAW" like booby traps, and at no time, -whether he is setting them up or watching them work, or activating them himself-, he shows any type of emotion on his face, except maybe the occasional smirk on his face.
Here is an edited video I put together to give you a better visual explanation of my theory:
To make matters worst, at the end of the movie, the family ONCE AGAIN show their ungratefulness towards Kevin. "Oh, but Buzz does says thanks to Kevin, and even lets him be the first one to open a present", you may say. And I will respond: "Nope. Buzz says:
Then Buzz suggests that Kevin should open the first present, and mom gives him a big kiss and a sweet "Oh Buzz", everybody claps and says merry Christmas to Kevin, and right after, Buzz says: "Ok. Enough of this gooey sh..stuff", and they all proceed to open their presents, except for Kevin.
One more small detail, what a SHITTY way to make it up to Kevin but the parents, the uncles, the cousins and siblings. They let the kid who wets the bed take the KING SIZE, while everybody else sleeps on the floor, including KEVIN??????? and the parents in the master bedroom, while the uncle and aunt have a private room as well????
And lastly, not happy with the free room upgrade to that HUGE suite, AND all the free presents from Mr. Duncan, the dad still has the nerve to get mad at Kevin for spending a measly $900+ in food? REALLY??????
So, with a family like that, a family that makes him feel unwanted, like he does not belong, like everything he does is a mistake, a family that does not acknowledge him or his achievements, no wonder he occupied his free and unsupervised time to study and research things such as how electricity travels, and figuring out that if you fill up a toilet bowl with flammable liquid and turn the water off, somebody with a head on fire will first try to open the water to put the fire out, but since there is no water, will try to dunk his head in the toilet and KABOOM!!!! Very advanced thinking and problem solving for a 9 year old, if you ask me.
I hope you enjoyed this theory, and thank you for reading!