Building on Kapua's theory that the bodies were thrown into the ocean, Steven would have done that only to get them as far away from his home as possible. Dismembering her would make it more likely that fish would feed on the remains and a whole body wouldn't be found.
I'm not ready to post what I think because I don't know what to think.
But I have given some thought to Charli's phone. Charli's phone is what guided Steven's alibi. If the Scott family hadn't received the ping on her last known location, they wouldn't have searched Honomanu right away. I know there's been some discussion about her body being in Haiku or "upcountry" but that's too close to home for him.
The simplest reason is usually the truthful reason. His or her phone may have gone off causing him to rush and get sloppy. I think he had completely forgotten about her phone being a beacon. Otherwise he could've tossed it out the window along the way. The road to Hana is dark and lonely at night. You can't see into the cars of oncoming traffic.
Why Honomanu? Because it's dense and dark and there's water nearby. Why was he allegedly seen in Hana? Because it's far away from his Haiku home.