You do know that Khashoggi was an American journalist right? And that the apparent motive for his killing was because of his actions as a responsible member of the free press?
Beyond that, this is absolutely a global matter. The world has a duty not to normalize the killing of residents of another country, especially when their job is to promote international transparency, and that's what gets them killed. Beyond that, he was killed at a goddamn consulate which is an international system for the safe conduct of business between nations. The world has an imperative to protect this. Saudi actions threaten the entire world here, and as such America would need to be a leader in this response. That would be the case if, for example, Khashoggi was instead a UK resident working for the Sunday Times.
But far beyond that. Even if this was entirely a matter fully contained within SA, the US does regularly and should continue to consider the private conduct of other nations with regard to our independent business with them. We apply sanctions to countries which violate human rights, and we use their conduct domestically and in foreign affairs to judge their suitability for trade and other international relations.
An analogy: you want to buy a used car off Craigslist. You find what seems to be a great deal on your dream car, right color and everything. So you run Carfax, make the call and everything appears on the up and up. You meet up with the guy for a test drive and things go great. So you make a handshake deal and head to the bank to get a cashier's check. On your way back to the neighborhood, his neighbor flags you down having seen you inspecting the car. He tells you not to buy it and that, although they've never had any actual problem with him, he is a registered sex offender.
What do you do? Are you going to buy the car with no further action because his sex crimes are completely unrelated? I'm not saying it's wrong to buy the car. But I think you would reasonable at that point consider getting more information about the car and the seller or to choose to pass altogether.