Make sure he is dead, delay any mind controled bystanders that suddenly feel compelled to help him, cut him to pieces, capture his soul, teleport to various places far away from each other, completely obliterate body parts, clean the bloodstains, burn his soul, THE FUCKING END!
Why torture him, its useless and only gives other puppets chance to save him. If you need to kill something, kill it. If it is unkillable, break its mind, kill his close ones, make his subjects live in poverty while they blame him. THAT is how you should do it.