Drystan knew that Klein was powerful and that all of his threats were not baseless. If he really wanted to get something, he would go to any lengths to do it even though the means he used was disgusting and inhumane he would still do it.
"I will tell Llewel everything, I don't want to continue keeping him in the dark since he is the next ruler of Verzalys this way he can prepare himself from the brewing storm that might befall us," this was a decision he made even before they were able to return. He could prepare Llewel to face the enemy and teach him how to deal with them because he felt that he was not going to live that long anymore.
"Are you going to reveal their existence to him as well?" Kane asked.
