I think this is a very perceptive comment.
It's nearly impossible to comprehend that JV and JMF adopted Preston solely to abuse him

but it must be the case because it's obvious that JV hates Preston.
Not only sexually abusing him but hitting, slapping, gripping and handling him roughly, breaking his arm

shouting at him and startling him and keeping him awake and spinning him too fast on that ride

And the way he looks at photos and footage of Preston, in that police interview where he has the blond hair, he's looking at him with absolute contempt

This is the baby that allegedly drowned accidentally, you'd be devastated looking at the photos, but there's not one ounce of sadness or regard for Preston

It's like once they got him into their "care" they expected him to be completely like a doll, compliant?
But instead he was "hard work", keeping them awake at night

So they hated him, not that they were ever going to love him - they wanted a slave, awful as that sounds, a compliant one

It was like an experiment of a sick fantasy that they had but it wasn't working out quite like they had planned.