Responses

They didn't destroy Cloyan kingdom (Marie's home) they concurred it. They killed the royalty and Marie went into hiding in fear they'll kill her too. Now the kingdom is unstable, hence marrying Morina will bring peace to Cloyan since she's like a Saint to the ppl. And Politically too marrying a Princess is better than marrying a Count or a Duke child.
They destroyed Marie's kingdom right? If they wanted to marry her they should have asked for her before they ever came to bring down her kingdom lol then she wouldn't have ran away....