Because they were once the richest business family. They were jealous of this rich family. Commissioner was from commoner compared to them being related to royal family.
The commitioner and his bastard predator son framed them for revenge and money. The commitioner was proven to be even more brutal by killing his cowardly son whom Jiusu took him hostage. His greed showed no end as he even beheaded his other kin who was Jiusu friend and blackmailed Yu Shai to work with him with his sister whom he secretly raped during her confinement
