If he was gonna kill her why talk about marriage with her?
Basically he just have to get the power of the jewel in his hands, so the "softer" method is just to marry her and monopolize her power and have her power be passed down his descendants, the other method is to take her soul ( that presumed to be the jewel thus also it's power ) and use the power himselfTo grab a vague reason to name himself king after he kills the rest of the royal family, that has to be the plan right.
To grab a vague reason to name himself king after he kills the rest of the royal family, that has to be the plan right.