You get what you pay for
...I believe it was a group of thugs and fools entering into a deal with Dewani without making an itemised list...
If they could have scraped together a single brain amongst the lot of them, the SA contigent would have relieved Dewani and his wife of their valuables, and pulled Dewani aside to offer the following advise:
"Shrien, you and I both know that the real reason you are not going to report this incident to the police is because you will then be mightily at risk that your murderous intentions will be exposed. Not to mention that even if the police don't believe us when we tell them that this was all YOUR idea, the story being out will put a crimp in your ability to come up with a Plan B to have her bumped off later on at another place and time of your choosing.
That will be the REAL reason you don't report this. But you'll need an excuse to give to the lovely Mrs. Dewani. So I'd suggest you tell her the reason you can't report it is because I at this moment am threatening to have my posse kill you and her if you do. So you've decided to cut your losses, thank God you both escaped with your lives, and head back to the safety of England ASAP".
It would have been a lucrative crime, and not only would they likely never have been caught for it, it probably would never even have been reported.
...there was nothing to stop these two murderers from killing Dewani as well. But they didn't. Why not?...
I don't agree with this at all. There were multiple reasons for not killing Dewani in that situation. Amongst the minor reasons is that Dewani still owed them more money.
But the major reason, and the one that gave him ample reason to believe he would be safe, is that Shrien Dewani had a role to play in throwing the cops off the scent. It was an important part of the cover up for Dewani to tell the police that it had been
his wife's idea to go slumming around the township.
If Shrien had not been there to tell the police it was all his wife's idea, the heat on the taxi driver would have been considerable. But Shrien's story helped paint the picture of an entirely random incident in which the taxi driver was as much of a victim as Shrien. And that was very helpful.
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