It remains to be seen whether any sanctions will actually occur, and whether they wind up being token sanctions or meaningful ones.
Some of the US political reaction might well be genuine because of just how brazen the Saudis seem to have been about this: in geopolitics it is normal to keep up the pretense of diplomatic niceties and interrogating and killing someone who has been lured in to an embassy breaks the conventions quite badly. But some of it might also be a reaction to the US media coverage.
The high profile of the story is being driven by Turkey, which has had rather unfriendly relations with Saudi Arabia for a while, and which has taken exception to Saudi Arabia abusing the diplomatic status of their Turkish embassy to murder Khashoggi (at least in the Turkish view: they have no doubt about what happened. Possibly because they've got the embassy well bugged). So Turkey keeps pushing out information about this case.
One truism of journalism is that stories about journalism and journalistic freedom are seen as vitally important by journalists. So the murder of a journalist because of what he wrote about the Saudi government is going to be headline news disproportionate to its actual significance in any event short of a major war breaking out. Which explains why this gets so much press coverage. And since the press is keeping the profile of this case high, politicians are forced to respond to it, because journalists are publicly asking them questions about it.
So because this generates a lot of media and public attention and some degree of shock, politicians are forced to respond appropriately. Hence there are lots of words about treating this very seriously, dire consequences for Saudi Arabia if it is proven etc. The words don't mean a great deal. It is the actions taken that signify what really matters. If it ends up being sanctions that are talked up a lot and amount to very little meaningful impact on anyone, then you know the politicians are trying to be seen to do something whilst sweeping it under the carpet as much as possible.
There is also domestic US politics involved in this too. Trump is pretty pro-Saudi, being on good business terms with them in the past. Some quotes from his campaign rallies in 2015 (four different quotes in one quote block).
“Saudi Arabia, I get along with all of them. They buy apartments from me. They spend $40 million, $50 million,”
Am I supposed to dislike them? I like them very much.”
“I make a lot of money from them.”
“They buy all sorts of my stuff. All kinds of toys from Trump. They pay me millions and hundred of millions.”
Consequently Trump's political enemies may well have seen an opportunity to damage him here, given his propensity for saying the "political" thing one minute and then undercutting it with something closer to his real opinion in more off the cuff moments. So there is the possibility of being able to stoke public outrage and expectations that the US should take a moral lead, and then they anticipate being able to contrast it with Trump trying very hard to avoid blaming the Saudis or taking any meaningful action against them.
As a further possibility, there may also be internal Saudi politics leaking in to the open. Crown prince Mohammad bin Salman (MBS) isn't the monarch, but at the moment he is for all intents and purposes the head of state and ultimately in charge. He won the Saudi power struggle in 2017 when King Salman replaced his previous heir presumptive Muhammad bin Nayef with MBS. MBS was seen as a reformer of sorts, with allowing women to drive, trying to modernize the kingdom to make it more technology focussed rather than being an oil state, and opposing the power of the old religious conservative establishment. Which has made him a lot of enemies. In November 2017 he arrested a large number of government ministers and princes and held them in a hotel for an extended period. Officially an anti-corruption drive, and possibly also a way of neutering some of his opposition.
But there is significant opposition to him within the Saudi establishment, and the modernizing vs conservative power struggle isn't entirely settled. So there may well be elements with Saudi Arabia helping to stir the pot in the Khashoggi case, and it is not impossible that they have back room deals with various groups in the US to try and leverage this to mutual benefit.
Yes, a lot of this is wild speculation, but that's life in geopolitics. The people who know what is driving the public events don't tend to talk about it until they write their memoirs decades later. But you can be pretty sure that the actually politically significant actions taken will be driven by something other than questions of morality, although they may be framed in those terms for public consumption.