Well as mentioned in the chapter, he is tired of the the same fate repeating for him and each time its miserable for him so he wants mc to atleast feels it ones what he's been going through. He knows mc has the dragon's orb so he wont die thats why he is doing it coz he wants mc to feel the agony of drowning while alive atleast ones. That mf still doesnt want t admit that what he is facing is consequences to his own actions. King never ordered to kill the mc coz he loves him but this mf took it on himself to kill the mc coz he was a thorn in the king's successful reign, king was mad and punished him by plucking out his eyeballs, cutting his tongue and throwing him in the sea. On top of that since he killed the dragon's lover, he got the curse to have the same life repeated again n again so now he wants to instinct atleast some of that pain back to mc coz he basically blames him for all that.
What's his purpose? Like what was their connection in the past? I didn't get much other than it was never really their fault. It was the evil's guidance blinding that man.