I really appreciate you asking because I always wondered too. I thought perhaps he was obsessed with Seonwoo and hurt Geumhoo to break them up. But then he seemed obsessed with Geumhoo and that's why he wanted to hurt Beom. But then he finds out Seonwoo is getting married and wants to get rid of Geomhoo because he doesn't care anymore?? He's the main villain of the story but his motivations are so bizarre and have no underlying reason.
But then he got so mad when Seonwoo and Geumhoo broke up in the past. And remember when he beat up Beom and asked him what does he have that Seonwoo doesn't. Is it that he was inlove with Seonwoo and he'd always want to see him smile and the only person who could do that is Geumhoo...? But then they broke up and that made him mad so he punished Geumhoo..?
Actually, can someone please explain to me in simple words why was the Prince obsessed with Geum Hoo and Seon Woo?? I am still a little confused about that