MP had plenty of motive to harm or kill SG
1) If she did not not get paid, as JB said he did not pay her; MP would be angry for not getting his driving fee (Usually 30%)
And he would get this money by killing her? Nope, but he would get this money by letting her alive and make clear what she owes him. So he had in fact motive to protect his "investment"
2) SG had called 911, if MP had in his possession contraband (drugs-weapons) he would be very angry the police were on the way (as far as he knew)
Which would demand that he knew about the 911 call in the first place. And if, wouldn't his first urge to get rid of the evidence or to leave with it? Which he actually did, because when police arrived, he had already left the area.
3) If MP was involved in some way with various Oak Beach residents in criminal activity, he would be angry SG called 911.
That is the conspiracy theory and I won't discuss that because there is no evidence, he had any contacts to Oak Beach prior to this visit. I mean, he had even to ask JB for the way as it looks.
4) The fact that SG was being difficult and not obeying MP's instructions could have provoked MP as well
Given, that he had her as customer several times before, that argument is as good as "he suddenly came to the idea, it would be nice to kill something else but time".
MP could have turned SG over to the doctor to 'treat' her.
And why would the doctor treat her with lethal consequences? How would MP know CPH? Why would MP do that in the first place, he had no motive?
Understand after driving all the way to Oak Beach from NYC, sitting in the car for hours, finding out SG is not going to pay you, that she has called 911, and she will not get in the car. Who would not be angry? People have died for less.
Aside of the fact, that he had no sufficient time window to kill her, this is as good as "sometimes people do bad things". Between angry and killing someone is a long road. Especially, if the actual killing is impossible in the time given.
It does not take a lot of provocation for some men to hurt women.
True, but then MP would have a hisroy of that. Any complaints about battery? Domestic violence? No. But the thing to abuse a passport to bring in another woman for prostitution. MP made a living with that, those women are his income base - but only as long as they are alive.
The reality is with SG dead, LE does not consider MP a POI. If SG could have told us the story of that night, MP might not be free.
Or he might would be free. But what we know is BB saw her, the police was there about ten minutes later and MP was already gone. That would give him 8 minutes to find her, kill her, carry her into the marsh, return to his car and leave. Lets do the math here:
1 min to find her if he is very lucky.
1 min to beat her over the head
5 minutes to carry her into the marsh
3 minutes back
1 min to the gate
and those times are all estimated as short as possible. One can argue, it's impossible to carry a dead body to the position, she was found, undress her and spread her belongings where they were found, in five minutes. But even with those unrealistic short five minutes, he would be at least 3 minutes short. Of course, if he didn't find her immediately, if he needed longer in the marsh, if he needed longer for the way back since he was not familiar with the area, the time he is short to be the hypothetical killer can be easily also 15 minutes or more. So, by all rules of mathematics and physics ... no way!