I'm guessing the ML was obsessed with the FML and poisoned his brother to make him "sickly" to try to get rid of him and break the engagement, and then when the wedding came up, he tricked the brother into meeting with him and he killed him or something so he could take his place as the groom.
I'm guessing the ML was obsessed with the FML and poisoned his brother to make him "sickly" to try to get rid of him and break the engagement, and then when the wedding came up, he tricked the brother into meeting with him and he killed him or something so he could take his place as the groom.