He wanted to give the corrupt noble, Vesta, a lesson. That's why he said Vesta's selfish actions led to losing a potentially high valuable ally. However, the later scene, in my opinion, clearly indicated he considers Rimuru right now a threat. He was probably almost happy he could use this path to get Rimuru out of his kingdom so smoothly. Even though Rimuru behaved, you could say it's even more scary. Only if you are feeling very safe and secure, you can just go with the flow openly. Like Rimuru showed absolutely no signs that he would be worried about severe sentences/punishments, like David said, as if he could break free and cause chaos any time he wanted. Not that it would be easy to try to judge a slime's feelings.