I don't think you understand his relationship with Sith.
They Invaded his adopted homelands, massacred a helpless world, he watch them kill children whilst fighting them, he watched his brothers in arms die before beasts and Sith personally on Concord Dawn, stopped them attempting to poison the waters of said world were innocents were everywhere, which were also being slaughtered, and then to top it all off they essentially nuked the land of his childhood.
He wants to watch them all burn biblically in an eternal pit of fire.