A threat of execution only controls someone who would rather live than lose their honour. A person who already accepts that they must die, and who counts dying for a duty as an honourable end, cannot be stopped by the punishment meant to deter them.
Some duties, like burying your own dead, rest on unwritten laws that no one remembers being made and that no ruler can repeal. A command from the state that orders you to break them has no real claim on you, however loudly it is enforced.
A ruler who treats every objection as a plot, or as bribery, cuts himself off from the warnings he most needs. Rule by one voice alone is not really a state at all, since a state is its people and not its master.