I would have otherwise thought he was helping the kids dispose of the body (and the evidence...without a body or a witness, it is circumstantial case...at least in the U.S.).
But life is stranger than fiction.
Joran is holding Natalee, and tells his Daddy that Natalee is going to report him for rape...which wouldn't do since Paulus is up for a judgeship. Daddy panicks and they kill Natalee just so she won't talk..
There are some blanks that need to be filled in, but it could be something along those lines?