I thought his best answer (most deceitful) was that he put her in the bird sanctuary. He knew that the government could not dig up the bird sanctuary looking for a body several years after she vanished. Investigators would be able to find a skull, but not without destroying the sanctuary.
I have no doubt that Natalie suffered the same fate as Stephanie. Stephanie said something that upset Joran and he hit her in the face with his elbow. With Natalie, I think it was when she said something about her mother's family history having some association with a plantation with slaves, or discussing Nazis. I think that set him off, and something about gambling poorly set him off with Stephanie. She was good at the casino, but Joran simply lost money, thousands of dollars at a time. Joran has no impulse control. If he feels angry, he lashes out. He is contrite later, but the lashing out does not seem to have any boundaries.
His parents tried to correct him, but seeing a psychologist during high school, and involvement in sports, did not Change Joran's behavior. It was when he threw someone off the bridge that he was first assigned a psychologist. He was also violent with his younger brothers. As far as I can understand, he was problematic from the time that the family moved to Aruba, when he was very young. I doubt that he would have been any different had he been raised in the Netherlands. His mother thinks that there is something wrong in his head, and I agree. He should be locked up for life and should not be allowed to marry.