What led to Jamal Khashoggi's murder?

The disappearance of prominent Saudi journalist Jamal Khashoggi shocked the world. It was Oct 2nd when Khashoggi was seen at the Saudi Arabian Consulate in Istanbul, Turkey. He entered, but reportedly never left. Khashoggi fled Saudi Arabia in September, 2017 and went into exile to the United States, where he joined Washington Post and became even more outspoken and openly criticized about the Saudi Arabia's military campaign in Yemen, as well as Mohammed Bin Salman's leadership. So what exactly led to his sudden disappearance and murder??