There is a very lenghty, but quite interesting article on ZH about the issue:
www.zerohedge.com/news/2018-10-12/killing-saudi-journalist-khashoggi-could-spell-end-mohammad-bin-salman
In very short:
This guy switched sides & allegiances so often, that he knew quite a lot. Was a protegee of Turki bin Faisal Al-Saud.
In Washington/CIA (re. Aghanistan/Mujahadeen), and out (during Bush era, who preferred Bandar bin Sultan)
Later Al-Waleed Bin Talal, Qatar, also Israel … but always as Advisor/Press&Media Handler (again CIA) – so my guess would be he really just was a thorn in Bin Salman's side and was killed.
There was purportedly a 15 man team flying in & out of Turkey before & right after the Oct 02 happening – not unlikely a hit I'd think.