In his public writings, Jamal Khashoggi's criticism of Saudi Arabia and its Crown Prince Mohammed bin Salman was measured. In private, the Washington Post columnist didn't hold back.

In more than 400 WhatsApp messages sent to a fellow Saudi exile in the year before he was killed at the Saudi consulate in Istanbul, Khashoggi describes bin Salman -- often referred to as MBS -- as a "beast," a "pac-man" who would devour all in his path, even his supporters.

CNN has been granted exclusive access to the correspondence between Khashoggi and Montreal-based activist Omar Abdulaziz. The messages shared by Abdulaziz, which include voice recordings, photos and videos, paint a picture of a man deeply troubled by what he regarded as the petulance of his kingdom's powerful young prince.

"The more victims he eats, the more he wants," says Khashoggi in one message sent in May, just after a group of Saudi activists had been rounded up. "I will not be surprised if the oppression will reach even those who are cheering him on."

The exchanges reveal a progression from talk to action -- the pair had begun planning an online youth movement that would hold the Saudi state to account. "[Jamal] believed that MBS is the issue, is the problem and he said this kid should be stopped," Abdulaziz said in an interview with CNN.

But in August, when he believed their conversations may have been intercepted by Saudi authorities, a sense of foreboding descends over Khashoggi. "God help us," he wrote.

Two months later, he was dead.

Abdulaziz on Sunday launched a lawsuit against an Israeli company that invented the software he believes was used to hack his phone.

"The hacking of my phone played a major role in what happened to Jamal, I am really sorry to say," Abdelaziz told CNN. "The guilt is killing me." (CNN)