The kid is a psychopath, pretty much textbook definition. He wants something, he figures out how to get it, he does it. No consideration for how others are affected. He wants to have the party, his parents object, he gets rid of them. Problem solved.
One of my friends is a cop, and he told me about a "test" they have, basically to see if you're a psycopath or have the potential to be one:
A man's sister dies. At the funeral, he meets a woman and they end up talking for a while. Afterwards, people are leaving and he tries to find the woman, but he doesn't see her, and realizes that he never found out her name. He asks around, but no one can tell him who she was. So he kills his brother.
Why?