Very, very sad for Charli's family. Though LE are still calling this a missing person's case, I have had a terrible feeling re: this case from the very beginning. While LE are not calling SC a suspect -only a POI- at this time, if he did indeed harm Charlie, it could have gone like this, IMO:
1. SC calls Charli. Tells her he is having some kind of car trouble (I'm not sure what kind, & SC seems pretty confused as well since he says -in a single interview!- "my truck that I got stuck," "I had extra light tools" & then finally "in case my truck broke down") & to pick him up
from his home (IMO, he didn't have a cell w/ him when his truck supposedly broke down because he didn't want to be tracked via 'phone, just in case he was to become a suspect -yes, I believe this was very much premeditated- & that is why he had to wait until he was home to call her).
2. I have not seen anything to indicate Charli told anyone prior to leaving her sister's house exactly where SC's truck was (MSM has even said, "Mile marker 20 on the Hana Highway was one of the last spots where Charli was seen," but this is according to SC only. No official word from LE to confirm this). I believe SC had driven to & parked his truck in Jaws -specifically, a spot near where Charli's SUV would eventually be found- earlier in the day & was given a ride home by an accomplice (Charli's mom: "(Charli's friends) said that
on a hunch because of where Steven lives, they decided to go down there and look and they found her car." Others have indicated Haiku -where SC lives- is pretty close to Jaws as well.)
3. Once Charli picks him up, SC informs her his truck is in the Jaws area & gives her directions. Once there, they both get out of her SUV, leaving Nala inside it, supposedly to work on his truck, but then he attacks her (perhaps SC even had a rope in his hand, supposed to tow the truck w/). SC has the same accomplice waiting for them there to assist him in the attack & the aftermath (e.g., putting the rope, etc. in a garbage bag, preparing the body for disposal, etc.). Once Charli is gone, SC moves Nala into his truck because most of the "dirty work" is done inside the SUV (thus burning it later to get rid of evidence).
4. I don't think he would've driven around the island w/ her body in a vehicle (the longer he has it, the more risk of being discovered). Is the spot where Charli's SUV was found close enough to the ocean that two people could carry & throw a body -say, wrapped in a tarp, secured with duct tape- directly into the ocean? Perhaps the body was driven in Charli's SUV to the northern edge of Jaws, at which time it was pushed off of a cliff?
5. At this point, SC -along w/ Nala ... who is fairly comfortable w/ SC because of his on-and-off relationship w/ Charli- leaves in his truck to go for a drive along the Hana Highway. He makes sure to toss Charlie's cell 'phone out at MM20 because that is where he intends to tell everyone Charlie was should he become a suspect (& that's where the last ping from her cell came from as far as we know). SC seems to have made MUCH effort (e.g., removing clothing from the body solely for the purpose of placing it somewhere away from the actual crime scene) in his attempt to throw LE off by scattering various evidence at multiple locations along the Highway. Finally, he drops Nala off near a shop in Hana, where she would be found in a matter of few hours. He had no reason to kill her because -unlike Charlie ...- he had no animosity toward her. This was not a crime of opportunity, IMO.
6. SC returns home (where he had left his cell 'phone ... now, he finally sends a text to Charlie's cell). In the meantime, his accomplice has been stripping parts off of Charlie's SUV. One of the mysteries re: this case to me has been why the SUV was found on its side. So I did some research & found this "device":
"Fixed-Height, Standard-Model Rotisserie lets you rotate body/frame assembly a full 360 degrees, and lock it in at any position"
http://www.eastwood.com/resto-rotisserie-standard-model-unpainted.html
Perhaps the accomplice had something like this with him/her, & SC let him/her keep the parts for helping him ....
7. Once he/she is done, the accomplice sets the SUV on fire & then leaves the scene.
Ugh, I just now realized ...: I think Charlie had her cell w/ her, so under this scenario, LE would be able to tell she went to Jaws before her 'phone ending up at MM20 ... hmm ....