The Jews killed by Hitler were not US citizens, nor were the Rwandan victims of genocide. US citizenship has never been a benchmark to measure an atrocity. It does not matter if Khashoggi was a US citizen or not.
In life, you don't get both. As Trump pointed out in 60 minutes, the Saudis buy quite a bit from America. Notwithstanding whats' happening in Yemen, I'd rather keep selling to Saudi and let them work out the Khasshogi thing and then put their money to work to helping the U.S.