What movie was about a boy who killed his parents then got adopted and killed his adopted parents?

I believe the movie is called "The Omen".

It wasn't The Omen. The parents in The Omen were the biological parents. I think the movie you're asking about is the 1992 movie called "Mikey". It stars Brain Bonsall of the tv series Family Ties. Check it out at imdb.com to see if it's the movie you're asking about

*correction* the movie the Omen Damien was adopted.