Comment by qubec on Come On Now, Even If I’m Not A Villain - Chapter 20

@qubec he wanted her because of her beauty, nobility & status as only Daughter of the Rubia Family that would had brought a lot of money to the Royal Family (therefore him), but he didn't care that she would die soon and used her need of magic to make sure she was forced to stay with him
it was a ruthless perfect plan, but the rebirth of the Princess in her body destroyed his plan

@Eustass90 Law Actually, I think he planned to have a proper relationship to be at least a half-decent person, but the true purpose was to pave the way for his childhood friend to marry him in the future. Basically, marry powerful woman to stabilize his reign -> once she dies, take his childhood friend as empress now that nobody will make a fuss