That photo paints a picture for me.
I think he confessed to a story. Credible but not really. I see that cinderblock, I see that dinghy...
It computes for that he overpowered her, despite her attempts to fight him off, and that he likely looked around for what he could use.
Grabbed a cinderblock (or the other half of a broken one, walked her body out to an anchored boat, reached deeper waters and used the cinderblock and whatever he had access to, for a tether.
It's truly hard to fathom that he was once someone's beloved newborn, toddler, cheeky kindergartener. His current self so devoid of decency, it's like he morphed into evil at a particular age or it was somehow there all along, just waiting to strike, deriving pleasure from (the) pain (he inflicts).
How do you go from Legoes to cinderblocks?
Utterly frightening.
JMO