Rebellion, this is a crime punishable by the execution of nine kin; Jiang Chen merely annihilated a family, which can already be considered quite merciful.
Of course, the main reason was that too many noble clans were involved in this incident. If the sentence was indeed execution of nine kin, then at least half of the noble families of the Human Race would have been wiped out.
Noble families intermarrying is too common, and they are all relatives to each other. If one were to investigate thoroughly and implicate nine kin, it would not be surprising if even the Jiang Family were implicated.
The involvement was too extensive; executing nine kin was clearly not practical, so exterminating a family became the best choice.
No sooner had Jiang Chen's order been issued than the void where the Ancestral Land lay started to twist, and soon after, several somewhat disheveled figures emerged from it.