Location and audacity. It was done in another sovereign country, one with string US and European ties, and which isn't currently in civil unrest.
Also, they practically took him into their embassy, killed him and then lied about it in the most haphazard way.
That represents a massive break from international status quo. Unfortunately getting involved in third-world civil wars and financing terrorist attacks is quite within the status quo.
I was wondering why they'd do this in their embassy, instead of just performing a hit. At first I thought it was kind of plausible that they just wanted to kidnap him, but that things escalated in an unscripted way.
But ... a friend raised an interesting point -- part of the stink around the Russian assassinations in the UK was that they were done on UK territory, and endangered uninvolved UK citizens. The Saudi embassy is Saudi territory. Seems like splitting hairs, but maybe this factored into their decision to do Kashoggi in the embassy.
EDIT> It's just so weird. If you're going to murder someone, you don't invite them over for dinner and do it in your living room, or am I stupid?
This is not a massive break from the international status quo by a any means. The USA and all other nations have operated in a similar manner for a long time with equally gruesome outcomes.
If I recall the head of Interpol “disappeared” not long ago, and that little kerfuffle died away right quick.
Perhaps the reasons are more mundane.
1. The Saudis got caught out by the Turks with the evidence. Which has been used effectively. The Turks have a lot to gain by seeing the alliance between SA and the US weakened.
2. The senior leadership of SA can be more directly tied to this incident, as opposed to more common case where plausible deniability exists. Rookie mistake.
3. There exists internal opposition to the leadership in the US and SA that find it convenient to escalate the profile of this issue for their own purposes.
This will blow over and business as usual will resume shortly.
The whole point of extraordinary rendition is to remove the target to another country to give some legal cover and plausible (hah) deniability to the subsequent interrogation / torture / what have you.
Murdering someone in your embassy (although technically your own sovereign territory) is a new low.
Also, they practically took him into their embassy, killed him and then lied about it in the most haphazard way.
That represents a massive break from international status quo. Unfortunately getting involved in third-world civil wars and financing terrorist attacks is quite within the status quo.