Here is the publicly available evidence: A bunch of Saudi intelligence agents and wetwork contractors flew into Turkey for the express purpose of murdering a political opponent of the Saudi regime, which they successfully did at the Saudi embassy. They then attempted to replace him with a body double to fool efforts to detect their crime. The crime itself was recorded on audio. Everyone knows exactly who murdered him, where, how, and why.
After doing this, they disposed of his body (which they still have not returned) and promptly lied dozens of times in a row about the circumstances of his disappearance and death. They originally pitched he had wandered out of the embassy. Then that he had disappeared unrelatedly. Then that he had died, but no one knows how. Then that it was after a fight in the embassy. Then eventually admitting that it was a targeted murder, but by rogue agents. At each step, they lied until their lies were publicly and definitively exposed, largely by the Turkish government who are understandably mad that a foreign government exploited the diplomatic easement given to embassies to murder someone on their soil.
In conversations following up on this affair, the position of the Saudi government has been simultaneously that Khashoggi was a dangerous anti-government subversive who no one should mourn because he deserved to be killed, that KSA should face no consequences and be the ones to determine what justice means, and that no one not being charged by the KSA could possibly be involved in the decision to do this.
We have two possibilities from this stage: Case 1: This was ordered by either the Crown Prince or someone near him; or Case 2: This was a wonderful act of serendipity that the Crown Prince thinks was a very good outcome, but which was independently pursued by Very Bad Men Who Will Be Executed, and surely they were acting alone.
How would we be able to tell between cases 1 and 2 (besides the fact that case 2 is completely implausible and is "Kid with Crumbs Around Mouth Claims Dog Broke In To Cookie Jar" nonsense)? Well, we'd know for sure that case 1 was true if for example we had access to the Crown Prince's private correspondence and it demonstrated that he was involved. It would be difficult to discern between the two without this. Okay, so we all agree that the next step in this investigation is someone who intercepted the Crown Prince's correspondence to confirm what it says.
Note that at this stage in my post, the United States and its intelligence community has not been mentioned and nothing said so far relies on its credibility. So, given this, would you care to make a case for Case 2? Would you care to hypothesize which institutions, globally, we might rely on to make Case 1?
The CIA and FBI suck and have agendas. Turkey sucks and has an agenda. KSA sucks and has an agenda. And yet we can do better than nihilism when it comes to adjudicating this situation.