Natuchipss's feed

Wait what does Muyeol mean with "a pity for your father who entrusted his son to the very nephew who would one day kill him"??? Let's rewind. Wasn't Seoho who imprisoned Bihwan? When did Muyeol kill him? Is he saying he'll kill Bihwan? And also, isn't Bihwan a Dokkaebi? Is Muyeol also a Dokkaebi since they're family?